The billable invoice now subclasses the Invoice base class.
The computations involved in preparation, namely filtering out
nonbillable projects and PIs, and validating PI names, have been
moved to `util.py`.

The function for applying the New-PI credit (`apply_credits_new_pi`)
is also moved there, and the I/O needed to read and write out
the PI file has been moved out of this functions for ease of testing.