Change Parabola inputs back to cx and cy (from fx and fy)

[taylorbrown75]

Switch parabola inputs to cx and cy (as in legacy SolTrace) rather than use focal lengths. Users can traditionally set cx or cy to zero, which results in inf focal lengths, which we do not want to use as input values.

Originally posted by @taylorbrown75 in #72 (comment)

[taylorbrown75]

https://github.com/NREL/SolTrace/tree/jm-dev-branch