Add support for more Phantom equations of state

[dmentipl]

Phantom has integer codes ieos that determine the equation of state (eos). These codes are written to the snapshot file header. These are required to calculate the temperature or pressure as these variables are not written to file. The meaning of the ieos codes can be inferred from the source code eos.F90.

Currently, Plonk supports:

• ieos=1: globally isothermal eos,
• ieos=2: adiabatic eos,
• ieos=3: vertically isothermal eos (appropriate for discs).

We should add support for more Phantom equations of state.

For reference, here is the list of values at the current version:

! This module contains stuff to do with the equation of state
!  Current options:
!     1 = isothermal eos
!     2 = adiabatic/polytropic eos
!     3 = eos for a locally isothermal disc as in Lodato & Pringle (2007)
!     4 = GR isothermal
!     6 = eos for a locally isothermal disc as in Lodato & Pringle (2007),
!         centered on a sink particle
!     7 = z-dependent locally isothermal eos
!     8 = Barotropic eos
!     9 = Piecewise polytrope
!    10 = MESA EoS
!    11 = isothermal eos with zero pressure
!    12 = ideal gas with radiation pressure
!    14 = locally isothermal prescription from Farris et al. (2014) for binary system
!    15 = Helmholtz free energy eos
!    16 = Shen eos
!    19 = Variable gamma (requires KROME)