Metadata handling for missing subject rows

[mstackhouse]

I didn't address this in #83 - and getting to the meat of it is a bit complicated.

Missing subjects creates two points of complication:

• The data being queried needs to be the population data
• The query is not just a filter, but pulling the distinct_by values themselves based on the filter.

A few extras are required with this:

• [ ] A tplyr_meta object needs to have a new field added of source for the dataset that's queried
• [ ] The filter will be specific to the list of subject numbers