Add getem_costs_eqn_calc and test

[dguittet]

Added src/GeothermalCosts_eqns.c

@mjprilliman The documentation, getem_om_cost_calc_doc, in the above file needs to be updated. This is what is displayed to the user. Note that the variable list here is different than in ssc/ssc/cmod_geothermal_costs_eqns.h because there are 5 input variables that are already within cmod_geothermal_costs. So instead of asking the user to input these, they are taken directly from the ssc_data of the GeothermalCosts module.

Example of how the equation works is in the test file below.

[sjanzou]

Awesome - working for me in local build of pysam. Thanks for all the pull requests and the steps to add an equation!