Consolidate feedback xfer function eval code

Provide a single, high-level function for computing any term present in
the transfer function of either control board feedback circuit
_(high-voltage or impedance)_.

The `compute_from_transfer_function` function in the
`dmf_control_board.calibrate.feedback` module computes the term
specified through the `solve_for` argument in terms of the value
assigned to each term provided as a keyword argument with the
corresponding name.  The optional `symbolic` keyword argument may be set
to `True` to return a symbolic `sympy` equality rather than an evaluated
numeric solution.