Wind tunnel setup and injection module

[themikelau]

New setup and injection module "windtunnel" for gas polytrope in a windtunnel. Input parameters include wind speed, wind pressure, wind density, sphere radius, and sphere mass.

• Note that I have changed the logic of set_star_density to "know" mass_is_set = .true. when massoftype(igas) > tiny(0.) (indifferent to the value of npart). This is needed for the wind tunnel, where the particle mass must be known and set before setting up the star, so that the wind can be initialised. An implication of this is that massoftype should not be initialised to zero in setup_binary, which seems to be unnecessary anyway.

Future work:

• Generalise the gas polytrope to any "star" that can already be set up using the setstar module
• Allow for arbitrary EoS, which is currently hard-wired to be an ideal gas EoS.
• Perhaps combine this with the BHL modules.