Map: Megastudy backend weighted stats master ticket

[d-callan]

This ticket will mostly serve as a place for me to keep track of associated sub-tasks, of which there may be many. We need to be able to accommodate in plot.data cases where one row in a data table does not represent a single 'record'. ex: the popbio megastudy sample entity contains both individuals and pooled samples. in order to average across these two types of records, we need to know how many individuals are in the pooled sample and do a weighted average. We also need to be able to accommodate cases where we need to impute 0s for tall data.

Things we need to do:
update line plot class api to know about row-dependent vocabularies and adjusted denominators (for 0 counts on tall data)
update group* fxns used by line plot for the same
update histogram plot class api for the same
update group fxns used by histo
then bar
then box
then cont table

i think all of these can have separate tickets, to keep some granular track of progress and give us the option to go live w/o a plot class/ floater type. priority order subject to change, individual tickets coming soon.

depends on #221
depends on #222
depends on #223
depends on #224
depends on #225