updated functions

[yucongalicechen]

• modified variable names and used DO for calculation
• The function compute_cve might still be a bit confusing because there is no cve variable (used muls=1/cve instead)

[yucongalicechen]

• Copied functions from absorption package to the start of functions.py (didn't change anything from the original absorption package - we might not want to change muls to cve because we need to calculate the sum of muls to get cve=1/sum(muls))
• Added docstring for set_distances_at_angle, set_muls_at_angle, compute_cve, and apply_corr