Clarify WTF 'VisitDate' actually means on each table.

My naming convention here is that VisitDate should always mean the date of
whatever the table is recording - so "VisitDate" on the preenrollment table
means the preenrollment date, on the followupfortb table it means the follow-up,
etc. Thus, all the places where VisitDate has been tacked on by the DMS guys
are now "EnrollDate" - since they came off of the master table.
Percolate "EnrollDate" through all the functions. Fixes #17.

One slight variation: I'm calling it EnrollDate on the master table, too,
so that its naming is consistent whether the DMS guys merged it onto a file
or I did.
Convert more Dates.  Fixes #15.
Make StudyId on master consistent with StudyID on everything else.
Now that StudyID comes on every table in the DMS export, mix is obsolete.

All it was doing was adding StudyId to each table. No longer necessary.
Closes #16.