Parameter file in snapshot

Sorry @pdraper.... one more for you...

This is an implementation of #140. The parameter file key-value pairs are written to the snapshots. 
Changes are:
 - Write the parameters to the snapshot.
 - Reduce the size of the parameter file structure by changing the maximal amount of lines it can contain.
 - Added an error message for when this limit is reached.
 - Change the type `lock_type` to `swift_lock_type` in `lock.h`

This last change is because `lock_type` is used in some MPI function prototype and our "#define" interferes with some of the includes if you are not careful about ordering. That solves the problem by naming the lock type in a unique way.

Apologies for the rather large changelog that this generates for 5 actual lines of useful code... 

See merge request !173

Conflicts:
	src/engine.c

fixdt kick changes.

Fixed the minimal SPH hydro version

Fix to #148. The code now runs without segfaulting when using the minimal SPH hydro code. Results are not great (by design) as the model is really poor (but simple).

Changes are:
 - A missing initialisation in `engine_init_particles()`.
 - A mistake in the calculation of the du/dt term.

See merge request !169

Kick task for fixdt and proper treatment of conserved quantities

This fixes #146 and #103.

 - In brief this adds a new task "kick_fixdt" for the case where we run with this policy. 
 - We then construct the type of kick task corresponding to this policy.
 - Some of the physics operations have been moved to separate files "drift.h", "kick.h" and "timestep.h" to simplify things and prevent redundancies.
 - Energy statistics are now computed at the end of the drift (and not of the kick any more) which allows to get a value even in a time-step not involving all particles. 
 - The frequency of the statistics is specified in the parameter file.

See merge request !167

Simplify, permit user control over affinity

This ensures we 'do the right thing' when the user imposes affinity through e.g. Intel MPI's I_MPI_PIN_DOMAIN or other mechanisms. Also no longer does works with a shuffled ```cpuid``` array, which will hopefully have less surprising failure modes, and means we don't have to handle hyperthreading explicitly. If we want to be clever in this way e.g. to maximise available cache, then we could replace libnuma with hwloc as discussed elsewhere.

Hopefully this gives us a reasonable affinity which is easily overridden using standard techniques.

See merge request !120

Added name of parameter file to error messages.



See merge request !170

Task plot improvements

I have updated the plotting script and the output of the task data such that we can now have the full information for a given time-step and not just the last call to `engine_launch()`. This means the `drift` tasks will now be included in the plots. They will also include the single-core part where the cells and tasks are reconstructed. 
The start and end time as well as the CPU frequency are dumped in order for this to work.

Also, added a cosmetic change to the output for the case where a snapshot gets dump. 

See merge request !166