Make sure all the GNU builtins for malloc and friends are switched off when compiling with tcmalloc and jemalloc.

Shadowswift repairs

See merge request !551

Add Pressure-Energy (P-U) SPH

Closes #197

See merge request !540

Added minor changes to remove debugging checks and to remove the extra hydro loop during initialisation

GIZMO MFM

See merge request !549

Moved Gizmo to GizmoMFV, and changed gizmo configuration option to gizmo-mfv. Made sure all unit tests compile again with gizmo-mfv.

Add generic utility function to find a value in an array

See merge request !547

Changed to using correct versions of expf and caching the result; unsure if this is correct still, though