CV Updates: units and MSRPOPL values

- Added <code>mins</code> and <code>%</code> as dropdown selectables for the unit of the OBSERV population
- Fixed bug that would cause OBSERV to save improperly if an MSRPOPL value was saved and then edited and re-saved with no value
- Updated MSRPOPL to act like EoC patient population, which stores integer values instead of checkbox values
- Added some preliminary styling for the unit selection on OBSERV