Define behavior when data does not have a measurement column

[grosscol]

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