Modeplot pc units

[CommonClimate]

This PR initially aimed to address #447, but ended up tackling a few minor things along the way, including:

• harmonizing how MultivarDecomp and SSARes objects call the original series
• inference of period_unit from time_unit in PSD (used to add an "s" at the end, which made sense for years but nothing else).
• • handling of offending time_name/unit combos like 'C.E' (not a unit) --> years CE.