reads cross sections from text file.
returns a dictionary with keys ‘kT’, ‘cross’.
kT = Temperature [eV]. array of values
cross = cross section [crossunits]. array of values
crossunits = conversion factor; cross * crossunits = cross section [cm^-2]. single value.
Text file should look like:
; any number of blank lines or lines with comments (starting with ‘;’),
; at any point in the file, are permitted.
; ALWAYS ignores any portion of a line after a ‘;’
1.0e-16 ; a single number. this is the “crossunits”.
; Cross sections in this file represent value [cm^-2] = (value in file) * crossunits
; … etc. Values above are just an example
; any nonzero amount of whitespace between values is allowed.