Define behavior when data does not have a measurement column

When there is no measure in the MAPT data:

... and there are no measures listed in the spek: Assume all observations are from an implicit measure.
... and there is one measure listed in the spek: Assume all observations are from that measure.
... and there are multiple measures listed in the spek: Emit error and halt.

The column uses for the MAPT data should be informed by the column specification of the input table of the spek.

Depends on #38

Display-Lab / bit-stomach