Updated chemistry code in inertgasbc experiment

Updated chemistry routines (inert_gas_chem.F) and added facility to
make_input_files_for_inert_gas_with_bc_model.m to generate initial
conditions based on solubility equilibrium.

Changes to be committed:

  modified:   ../matlab/make_input_files_for_inert_gas_with_bc_model.m
  deleted:    external_bc_test.c
  modified:   inert_gas_chem.F