First attempt at sub-grid physics, shapy, extra side programs and general code cleanup.

This version incorporates all extra features that already existed on a private UGent repository and have now been made public. This includes a first attempt at implementing radiative cooling, star formation and stellar feedback, the inclusion of a number of Python modules that exploit parts of the code (named shapy - Shadowfax Python modules), and a number of side programs: a program to perform a single Lloyd iteration on a set of input points, a test implementation of GIZMO (Hopkins, 2015), and a test implementation of TENET (Schaal, 2015). The code also features co-moving integration based on the implementation in Gadget2 (Springel, 2005), but this only works for the collisionless component at the moment.

Apart from this, a number of bugs have been fixed in the vacuum Riemann solver and other parts of the algorithm, and all code files have been cleaned up. The dependencies have been cleaned up as well, by limiting the number of included header files to what is strictly necessary using iwyu.

A number of added features has not been thoroughly tested: some Python modules might be broken, we still need to add a test for the stellar feedback, the GIZMO and TENET implementations are not fit for production usage, and full-blown simulations including gas, dark matter, star formation, stellar feedback and cooling are known to crash. These issues will be sorted out for our next release. If anything else does not behave as expected. please create a corresponding issue to let us know.