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.

Vectorise kernel

Should fix issue #154. Includes updates to `testKernel.c` that test `kernel_deval_vec` against the serial `kernel_deval`.
I have tested it with Cubic Spline, Wendland C2 + C6 kernels.

See merge request !150

Made the kernel test part of the testing suite and checks that the vector/scalar values are consistent.

Put all the hydro task dependencies linking into a single function.

This implements @tt 's suggestion to get all the hydro-task dependency construction in one single place. 

See merge request !152