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.

Added documentation for the minimal SPH implementation such that it can be used as a template for new versions of SPH.