Minor tweaks with prep_outcomes/calculate_outcomes

[vineetbansal]

prep_outcomes:

Be explicit in documentation - "Maximum number of molecules per frequency range" (also change default to 50000 and indicate in help).

https://github.com/vineetbansal/CLM/blob/4cc651b57a8f4f2da4514fc7e36586c85a82589c/src/clm/commands/inner_prep_outcomes_freq.py#L12

--output-file should have .csv extension

calculate_outcomes:

• max_orig_mols should be None by default (also add docs on what this actually is).
• extension for output_file in help docs