Incorporate Medicare and Medicaid/Medicare into the elig_timevar_collapse function

[alastairmatheson]

Some outstanding improvements remain for the elig_timevar_collapse function:

• Add in column names for mcaid_mcare and add mcare-only tables (and tweak function query to work on those tables).
• Add the contiguous column back in (i.e., identify if the person have continuous coverage over a period but there was a new row/new from_ and to_dates because of a change in one of the selected columns).
• Have a Shiny interface to just check the boxes of desired columns (this is a long-term goal).