Refactor get and putColumn

I've come to realize that getColumn is inherently type unstable.
Therefore I want to expose getColumn!, which forces the user to specify
the type and shape of the column.
Export getColumn!
Remove the general getColumn/putColumn for MeasurementSets

I want to guarantee type stability with the MeasurementSet interface.
More MeasurementSet tests
Merge pull request #7 from mweastwood/improvements

Type stability fixes and more tests