Planetary ideal gas

Add an ideal gas equation of state for the Planetary hydro scheme and add tests for comparison with Minimal. Note that slightly different results come from the slightly higher viscosity beta, but with Minimal's beta the results match.

doc/RTD/source/Planetary/equations_of_state.rst

@@ -20,9 +20,11 @@ to check the regions of validity.
 
 So far, we have implemented several Tillotson, ANEOS, SESAME, 
 and Hubbard \& MacFarlane (1980) materials, with more on the way.
-The material's ID is set by a base type ID (multiplied by 100), 
-plus a minor type:
+The material's ID is set by a somewhat arbitrary base type ID 
+(multiplied by 100) plus an individual value:
 
++ Ideal gas: ``0``
+    + Default (\\(\\gamma\\) set using ``--with-adiabatic-index``, default 5/3): ``0``
 + Tillotson (Melosh, 2007): ``1``
     + Iron: ``100``
     + Granite: ``101``
@@ -42,7 +44,7 @@ plus a minor type:
     + Fe85Si15 (Stewart, zenodo.org/record/3866550): ``402``
     
 The data files for the tabulated EoS can be downloaded using 
-the ``examples/EoSTables/get_eos_tables.sh`` script.
+the ``examples/Planetary/EoSTables/get_eos_tables.sh`` script.
 
 To enable one or multiple EoS, the corresponding ``planetary_use_*:``
 flag(s) must be set to ``1`` in the parameter file for a simulation,
@@ -57,7 +59,7 @@ do not always include transformations between the internal energy,
 temperature, and entropy. At the moment, we have implemented 
 \\(P(\\rho, u)\\) and \\(c_s(\\rho, u)\\), 
 which is sufficient for the :ref:`planetary_sph` hydro scheme, 
-but makes these materials currently incompatible with entropy-based schemes.
+but makes most materials currently incompatible with entropy-based schemes.
 
 The Tillotson sound speed was derived using 
 \\(c_s^2 = \\left. ( \\partial P / \\partial \\rho ) \\right|_S \\)
@@ -66,3 +68,5 @@ as described in
 Note that there is a typo in the sign of
 \\(du = T dS - P dV = T dS + (P / \\rho^2) d\\rho \\) in the appendix;
 the correct version was used in the derivation.
+
+The ideal gas uses the same equations detailed in :ref:`equation_of_state`.