Timed outputs

This implements the improvement #142.

The time of the first output and the time difference between subsequent outputs can be specified in the parameter file. So no dumps after a fix number of steps any more.

Users can also specify a unit system for the snapshots. That gets passed along to the routines dumping the snapshot. 

I have updated all the YAML run scripts as well.

See merge request !159

Fix parameter file parsing for duplicate names

The parameter file parser now checks for duplicate parameter names within sections and standalone parameters. It also checks for duplicate section names.

See merge request !161

Removed an extra kernel_igamma from the rho_dh calculation

There was an extra kernel_igamma in the calculation of rho_sh which was causing an offset to equivalent Gadget-2 runs. 

See merge request !162

Added check for duplicate parameters, both within a section and standalone and check for duplicate sections.

Exchange all foreign particles

Should fix #93.

See merge request !158

Commented out, note gparts test is unchecked.

When moving foreign particles to the end of the list, make sure we don't swap with one that is a foreign particle.

Proper name for entropy variable in Gadget-2 output

Fix to #151. 

See merge request !157

Time the initialisation step as well.

In times of difficult debugging that's the most minor change ever. The 0th time-step now also has an output displaying the wallclock time spent.

See merge request !156

Faster compilation

In the current master, every time we edit a file, we re-generate version.h which then triggers recompilation of a lot of file that include it. 

Here we re-generate a version_string.h instead that is only included in version.c. That speeds up the compilation when only a few files are modified.

See merge request !155

Mpi multi dt fix

How about this as a fix for #124 ? 
The cell properties relevant for the density and force calculation are updated on the receiving side by re-constructing them from the particles.

See merge request !153

Shuffle array

Added a function to randomly shuffle an array of particles and moved random_uniform into `tools.c`.

See merge request !154

Added function to shuffle an array of particles randomly. Moved random_uniform from testPair/test27cells to tools.c.