Wind test including radiation force + ieos = 17 + bug fixes

[lsiess]

Type of PR:
Bug fix / modification to existing code (fixes #540)

Description: Include a new series of tests in the test_wind module :

• check total internal and kinetic energy of the particles after the test
• test wind problem when radiation force + Bowen opacity are included

Implement ieos = 17 to keep gamma constant but allow the mean molecular weight to change according to H2 formation

Bug fixes

• case ieos = 5 was restricted to simulations with nucleation. It can now be used in any setup (same for ieos=17)
• fix bug in temperature evaluation in calc_muGamma

Testing: run simple wind models with ieos=5,17 to check that mu and gamma where not held constant

Did you run the bots? no

Did you update relevant documentation in the docs directory? no

[danieljprice]

this all looks good, remaining failure is just that the tolerance needs to be a bit higher for the internal energy:

 checking wind mass loss rate...........OK     [max err = 1.330E-16, tol = 2.220E-16]
 checking total internal energy.........FAILED [got  2.218E+02 should be   2.218E+02, err = 8.584E-15, tol = 5.000E-15]