Cleaned up GIZMO code, added SineWavePotential tests.

The GIZMO code should now be clean enough to merge with `master`, and to be used in scaling tests.

I have added 1-3D versions of a new SineWavePotential test, which tests gravitational hydrostatic equilibrium in an unphysical but convenient 1D periodic sine wave potential. This allows testing gravity + GIZMO hydro without running into boundary condition problems. Unfortunately, the non conservative form of the potential makes it impossible to test energy conservation in these tests. GIZMO can keep all three set ups stable with both an isothermal and an ideal gas equation of state. I have currently only tested with the exact Riemann solver, but the approximate solvers should work as well.

The code is still not stable enough to cope with the hydrostatic and disc patch tests; these are next on my list.

See merge request !317

Removed ideal gas specific SineWavePotential scripts and incorporated them in the isothermal script. Ideal gas now seems to work.

Added 2D and 3D SineWavePotential tests. Need to run them at higher resolution and for longer, but they seem to produce reasonable results.

Added a maximal smoothing length that can be used to prevent particles from becoming too big in vacuum cases near the box edges. Will break the accuracy of the hydrodynamic scheme but is necessary in some situations.