Planetary documentation

Add general documentation for planetary simulations.

More to be added but it's a start.

Closes #659 (closed)

doc/RTD/source/EquationOfState/index.rst

 .. Equations of State
    Loic Hausammann, 6th April 2018
-   Jacob Kegerreis, 3rd February 2019
+   Jacob Kegerreis, 13th March 2020
 
 .. _equation_of_state:
 
@@ -50,46 +50,13 @@ density-entropy scheme or the internal enegy in a density-energy SPH
 formulation) written to the snapshots is meaningless. The pressure,
 however, is always correct in all scheme.
 
+
+
 Planetary EoS
 -------------
-Configuring SWIFT with the ``--with-equation-of-state=planetary`` and 
-``--with-hydro=planetary`` options enables the use of multiple EoS.
-Every SPH particle then requires and carries the additional ``MaterialID`` flag 
-from the initial conditions file. This flag indicates the particle's material 
-and which EoS it should use. 
-
-So far, we have implemented several Tillotson, SESAME, and Hubbard \& MacFarlane 
-(1980) materials, with more on their way.
-The material's ID is set by a base type ID (multiplied by 100), plus a minor 
-type:
-
-+ Tillotson (Melosh, 2007): ``1``
-    + Iron: ``100``
-    + Granite: ``101``
-    + Water: ``102``
-+ Hubbard \& MacFarlane (1980): ``2``
-    + Hydrogen-helium atmosphere: ``200``
-    + Ice H20-CH4-NH3 mix: ``201``
-    + Rock SiO2-MgO-FeS-FeO mix: ``202``
-+ SESAME (and similar): ``3``
-    + Iron (2140): ``300``
-    + Basalt (7530): ``301``
-    + Water (7154): ``302``
-    + Senft \& Stewart (2008) water (in a SESAME-style table): ``303``
-
-Unlike the EoS for an ideal or isothermal gas, these more complicated materials 
-do not always include transformations between the internal energy, 
-temperature, and entropy. At the moment, we have only implemented 
-\\(P(\\rho, u)\\) and \\(c_s(\\rho, u)\\). 
-This is sufficient for the simple :ref:`planetary_sph` hydrodynamics scheme, 
-but makes these materials currently incompatible with other entropy-based 
-schemes.
-
-The Tillotson sound speed was derived using 
-\\(c_s^2 = \\left. \\dfrac{\\partial P}{\\partial \\rho} \\right|_S \\)
-as described in Kegerreis et al. (2019).
-The table files for the HM80 and SESAME-style EoS can be downloaded using 
-the ``examples/EoSTables/get_eos_tables.sh`` script.
+
+See :ref:`planetary_eos`.
+
 
 
 How to Implement a New Equation of State