Add calculation of occupations to free_fermi_proj

  - Calculate state occupations in free_fermi_proj.f90. This is something that
    would be hard to do using the recursion-relation code.

  - Checked for correctness by calculating the energy and particle number.

  - This version also includes a non-functioning heat capacity
    calculation. Trouble with Wick's theorem.
Use a function for computing the s.p. energies (free_fermi_proj)
Add some test data; update test scripts
Add a few comments