Integer timeline

This modified the way we do multiple time-steps. The time-steps are now placed on an integer timeline and can take values from 0 to 2^29 where 0 corresponds to the starting point of the simulation and 2^29 is the end point of the simulation. This effectively limits the number of time-steps that can be performed and sets an absolute minimal time-step size. If need be we can later on extend the number of time-steps allowed by switching to a long long but I have not seen this being necessary in any EAGLE or other simulations we ran.
However, it prevents the assignment of 0-sized time-steps that occur in the current version of the code when using a final time which is not of the form 2^n (n positive or negative integer). This hence fixes issue #87.
Quite a lot of changes scattered across the code but the only non-trivial ones are in runner_dokick() and engine_init().


While working on it I have corrected the count of number of particles updated in a given time-step. This is now correct. Fixes issue #89.

Finally, I have also added a message at start-up displaying which version of SPH we run.



See merge request !94

Missing files and other small bugs

A bit of a mix:

- Added files to the Makefile.am such that 'make dist' works (issue #100)
- Unit tests now compile (issue #99)
- Fixed XMF i/o crash (issue #101)



See merge request !93

Improved multi-timestep SPH

This brings a range of improvement, mostly minor and of code clarity nature to the code. 
It solves:

- Issue #84. Particles are now <128 Bytes in size.
- Issue #85. Smoothing-lengths now evolve more accurately and we implement a time-step criterion more conservative than default Gadget-2 but that is part of EAGLE.
- Issue #88. We now have a 'minimal' hydro implementation that can be used both as a template and an example. The doxygen documentation is based on that version. The documentation is now very extensive. This model is a poor SPH implementation and should only be used to understand the structure of the code and not be seriously run.
- Issue #95. The engine_collect_kick() function now correctly skips empty cells.
- Issue #96. Conserved quantities are now written to a file. We need gravity before this really get useful.
- The description of the SPH flavour in the HDF5 files is now moved to the hydro_io.h files for consistency.


Most of it are straight-forward changes. Note that this does not fix the issue we have in MPI mode.

See merge request !90

Engine policies for gravity and improved task mask construction

This is a straightforward change to engine_step() and engine_init_particles() that should allow other users to add or remove tasks without having to comment-out portions of the code. 

The right list of policies simply has to be passed to the engine when constructed. I have added three policies (hydro, external gravity and self gravity) to be future proof.

I have also resurrected the two functions that print the content of a mask. These had disappeared following a merge. Finally, some more robust bit-mask checking tests are done when testing policies. 

That should allow @jregan and @tt to add their external gravity tasks to the mask on line 1901 (in engine_step()) of engine.c without having to comment-out other parts of the code.

See merge request !91

Merged the tiny changes pushed to master.