Add support for equations of state related to planetary physics

Main changes are the (pretty isolated) new hydro scheme MinimalMultiMat and new equation of state planetary, along with examples etc. See the wiki for more details.

doc/Doxyfile.in

@@ -765,6 +765,7 @@ INPUT		       += @top_srcdir@/src/gravity/Default
 INPUT		       += @top_srcdir@/src/stars/Default
 INPUT		       += @top_srcdir@/src/riemann
 INPUT		       += @top_srcdir@/src/potential/point_mass
+INPUT		       += @top_srcdir@/src/equation_of_state/ideal_gas
 INPUT		       += @top_srcdir@/src/cooling/none
 INPUT		       += @top_srcdir@/src/chemistry/none