Specified private variables in class Crust
minor changes
Implemented Ode_Int_Delegate
Fixed headers
Added envelope class which calculates envelope models (code used to be in makegrid.cc)
Added options to envelope to use potek eos or conductivities
Fixed a bug in envelope make_grid

Also moved Ode delegate setting to the Ode constructor
Added flag to eos and envelope to use the Potek opacities