Add the cooling infrastructure and the const_du and const_lambda

@dc-arri1 I have opened this merge request so that we can push the main changes into the master branch to make everything available to everyone.

I'll have a few questions for you here. I'd be great if you could answer them so that I can merge things.

See merge request !242

Added the new cooling parameters to the parameter documentation. Renamed the parameters to differentiate them.

Pack the task types and re-arrange the task structure to go from 80 bytes to 64 bytes.



See merge request !238

Only relink parts and gparts when we actually have parts and gparts

Any reason not do this ? We spend some time linking things even when we don't have anything to link. That would speed up `space_rebuild()` and `cell_split()` in the cases without gravity, the latter of those to two being a scalar part of the code this could be quite important.

See merge request !234

Stop integer overflows in scheduler

Fix a couple of integer overflows and stop no gravity particles being considered
as valid.

See merge request !237

fix : missing include file in hydro_iact.h

Compiling on our clusters with the following configure line : 
```
$ ../sources/configure CC=mpicc CFLAGS=-I/ssoft/spack/lafnetscha/opt/spack/x86_E5v2_IntelIB/gcc-5.3.0/metis-5.1.0-zg7ap5yg7nqhkgsu7njnivar7lz3oy4z/include --prefix=/home/culpo/SWIFT/build/../install --enable-optimization --enable-parallel-hdf5 --enable-mpi --with-metis=/ssoft/spack/lafnetscha/opt/spack/x86_E5v2_IntelIB/gcc-5.3.0/metis-5.1.0-zg7ap5yg7nqhkgsu7njnivar7lz3oy4z
```
we get the following compilation error : 
```
libtool: compile:  mpicc -DHAVE_CONFIG_H -I. -I../../sources/src -I.. -DTIMER -I/ssoft/spack/lafnetscha/opt/spack/x86_E5v2_IntelIB/gcc-5.3.0/szip-2.1-luyic7zykee3mod772n6nixm3ki3ok2o/include -I/ssoft/spack/lafnetscha/opt/spack/x86_E5v2_IntelIB/gcc-5.3.0/zlib-1.2.8-2febcaqqgk6rskb6kw7vljlxjg5dra32/include -I/ssoft/spack/lafnetscha/opt/spack/x86_E5v2_IntelIB/gcc-5.3.0/hdf5-1.8.16-phuidcem3zjts6cynahfbqs3p3zerual/include -I/ssoft/spack/lafnetscha/opt/spack/x86_E5v2_IntelIB/gcc-5.3.0/metis-5.1.0-zg7ap5yg7nqhkgsu7njnivar7lz3oy4z/include -O3 -fomit-frame-pointer -malign-double -fstrict-aliasing -ffast-math -march=ivybridge -mavx -Wall -Wextra -Wno-unused-parameter -Werror -MT libswiftsim_la-runner.lo -MD -MP -MF .deps/libswiftsim_la-runner.Tpo -c ../../sources/src/runner.c  -fPIC -DPIC -o .libs/libswiftsim_la-runner.o
In file included from ../../sources/src/hydro.h:35:0,
                 from ../../sources/src/drift.h:29,
                 from ../../sources/src/runner.c:46:
../../sources/src/./hydro/Gadget2/hydro_iact.h: In function ‘runner_iact_vec_force’:
../../sources/src/./hydro/Gadget2/hydro_iact.h:644:26: error: implicit declaration of function ‘max’ [-Werror=implicit-function-declaration]
     pi[k]->force.v_sig = max(pi[k]->force.v_sig, v_sig.f[k]);
                          ^
```
This small PR seems to fix the issue.

See merge request !239

Get master building for now

Replaced all calls to fminf and fmaxf by macros

As the Intel compiler can't handle these properly, here is a replacement of all calls to `fmaxf` and `fminf` by a macro that just encapsulates the ternary. This produces assembly call to `vmaxpss` and `vminpss` as expected. 

Any reason not to do this ?

See merge request !235

Stop integer overflow in do_sub test and make sure no particles are considered as a no-op not a trigger.

Fix a trivial scaling bug in the task weights (could also overflow)

Cooling infrastructure

See discussion in email:

 - Adopted a common infrastructure for cooling.
 - Moved the "constant cooling" to the new infrastructure.
 - Created the files for "Creasey cooling". Still need fully working implementation.

See merge request !233

Documentation of the space and new names for the cells

Some small improvements (hopefully !) to the code documentation and variable descriptions. Implements #208.

See merge request !231

Replaced all calls to fminf and fmaxf by the macros that was added for the integer min/max functions.

cell->super for gravity-related business

I think we need to do this to make sure the gravity tasks are hooked at the right place. Given that the particles and g-particles can be distributed in a different fashion, we should not use the same `super` pointer as tasks might be defined at a different level in the tree. 

What do you think ?

See merge request !226

GCC 5x doesn't like this now we have -Wextra, if offends make count size_t as well