Update the IPO support for GCC and CLANG.

Closes #539

See merge request !735

Note that you may need to define "AR", "LD" and "RANLIB", if the names do not
match the default ones (clang/llvm are often trailed by version numbers).

Star formation following Schaye08

See merge request !705

Add some comments to the random number generator to make sure we don't change the numbers by mistake.

Planetary documentation

See merge request !733

Removed duplicate line in the script dowloading planetary EoS files. End of lines in the Uranus REAMDEs.

This should have been done as it includes the system flags for selecting POSIX, for instance

Removed the dependency upon pysphviewer in the solution script for the star-forming isolated galaxy.

Update the other hydro schemes to also compile with the star formation model and entropy floor model.

When recovering the mass of a particle, use the 'hydro_get_mass()' function such that the subgrid model also works for gizmoid schemes.