Refactor age-1 (and other group) partitioning of NASC, abundance, and biomass

[brandynlucca]

This PR includes:

• Refactored function for partitioning overall NASC/abundance/biomass and specific groups (e.g. sex) based on proportions calculated for age-1 fish per stratum
• Minor update to the stratum averaging function to ensure index names are retained
• New functions that appropriately compute not only abundance/biomass but also divvying them up by defined groups, e.g. sex, age
• Test, fixtures, and docstrings for all new functions

This PR overlaps with #355. This was done for easier downstream testing to make sure that there were no ill effects from the updated code.

[brandynlucca]

This integrates responses to suggested edits and comments from #355 with those specific to this open PR.

Relevant changes:

• transect.set_interval_distance -> utils.compute_interval_distance [efa5d9]
• Change number_proportions argument for get_proportions.get_weight_proportions_slice to require a Dict[str, pandas.DataFrame] [2a5358]
• Add exclude_filter as an argument to all get_*_proportions_slice functions [63501b]
• New functions for converting number densities to abundance/biomass, and partitioning values based on the age-1 (or whatever other group) slices [e6d3e9], [4a2450], [925e70]
• Removed set_population_metrics as this was deprecated by biology.set_abundance and biology.set_biomass [f13c2e]
• biology.set_abundance/biology.set_biomass -> biology.compute_abundance/biology.compute_biomass [576665]
• Incorporated suggested function docstring changes [de27de], [78a3b8], [d76889], [4b9aa1], [8571e2]
• Updates to feat_hake.py to reflect incorporated changes [cfc56e]

[leewujung]

I made a few inline comments that require some changes, but I think this PR is ready to be merged once those are addressed.

I made a note in #369 (comment) for incorporating xarray ND representation that will make the calculations more straightforward for the various slicing operations.