Io unit conversion

This implements two changes:

 - Unit conversion from ICs to internal units and from internal units to snapshots when doing i/o.
 - A much simpler interface to add/remove fields to be written/read during i/o.

Could you let me know whether your usual tests still run with this ? 

See merge request !194

Conflicts:
        src/Makefile.am
	src/parallel_io.c
	src/serial_io.c
	src/single_io.c

Gamma power speed up

"Fix" to #185. See also the discussion in #166.

Two improvements:

 - Speed-up the calculation of x^\gamma, x^{\gamma-1} and x^-(\gamma-1) . Currently support \gamma=5/3, \gamma=4/3 and \gamma=2.
 - Write the details of the hydro scheme in the snapshots.

Branch can be removed.

See merge request !195

This reverts commit 9f311bbc

Macro vectorisation

Created a macro called `WITH_VECTORIZATION` in `config.h` that enables vectorisation throughout the code. Replaced all occurrences of `VECTORIZE` with `WITH_VECTORIZATION` throughout the code. Replaced compiler macros, i.e `__AVX__`,`__MIC__` with platform defined macros: `HAVE_AVX`, `HAVE_AVX512_F`... in `vector.h`.

Added place holders for vectorised versions of force and density interactions for Gadget2 and Minimal SPH schemes. An error will be displayed at runtime if the code is compiled with vectorisation enabled and using either Gadget2 or Minimal SPH schemes.

See merge request !200

a few fixes to permit out of source builds for unit tests

There are still issues to be fixed in the tests themselves, see !198

See merge request !199

Added place holders for the vectorised versions of force and density interactions for Gadget2 and Minimal SPH schemes.

Merge branch 'qa/out_of_source_build' of gitlab.cosma.dur.ac.uk:swift/swiftsim into qa/out_of_source_build

Conflicts:
	tests/testReading.c

Scaling plots

Added code to create an extra output file that contains timing information for every timestep. The output file also contains a header that includes: branch, Git revision, compiler version, number of threads, number of MPI ranks, hydro scheme, hydro kernel, no. of neighbours and eta.

Added two parameters to cosmoVolume.yml that are used to create a file for the energy output per timestep and timing information.

Also added a python script that plots scaling results for varying numbers of threads and data series.

See merge request !197

enable out of source builds

The changes should be GNU-Make / Autotools compatible. On the machines I have access to they enable out of source builds, for instance :
```
mculpo@nuvolari:~/epfl/swift$ tree -d -L 2
.
├── build
│   ├── doc
│   ├── examples
│   ├── src
│   └── tests
├── install
│   ├── bin
│   ├── include
│   └── lib
└── sources
    ├── autom4te.cache
    ├── doc
    ├── examples
    ├── m4
    ├── src
    ├── tests
    └── theory
```
It enables having different build configurations pointing at the same source checkout. 

See merge request !198

Removed all compiler macros for vectorisation and replace with HAVE_ macros. Added WITH_VECTORIZATION #ifdef for turning vectorisation on and off.