Colleen's IDL code for HEROES gain corrections

This code includes:
(1) Code to fit Americium scan lines.
(2) Code that incorporates gain variations from Am scans and then fits fully-illuminated lines from the variable energy source.
(3) Code that selects flight data time intervals when detector voltages were at nominal values.
(4) Code that fits Cd109 lines to flight data vs time.
(5) apply_all_flight_gain_corrections - which applies the above corrections and makes a fits file if requested.
(6) make_gain_corrected_flight_files - loops through flight data and runs #5.
(7) plot_solar_data - sums up all gain corrected solar data without any aspect correction.