Gizmo refactoring

See merge request !967

Logger: fix intel compilation

See merge request !978

Planetary examples

Closes #633

See merge request !976

Logger add incref

See merge request !974

Removed GIZMO scheme specific hydro_g/setters.h and moved their contents to hydro_flux.h. Moved the centroid calculation to hydro_velocities.h and removed centroid variables from MFM particle.

Streamlined names of hydro variables in GIZMO schemes. Removed almost all overlap between MFM and MFV. Still need to get rid of some ifdefs in hydro.h.

Logger restart

See merge request !969

Logger: fix sanitizer

See merge request !973

Logger dimensionality compilation error fix

See merge request !972

Made sure logger_particle arrays have size 3 to fix a compilation error when configuring with hydro dimension =/= 3.

Fixed swiftsimio movie script in kelvinhelmholtz

See merge request !968

Logger index file

See merge request !813

Refactoring GIZMO schemes to avoid code duplication, step 1: moved MFM and MFV to subdirectories of new Gizmo directory, made common version of hydro_*.h included from rest of code that (for now) simply calls the right version.