Add a data method to check variables form a data frame

[davidorme]

Both the plants and animal models need to handle cohort data provided as a data frame (see #656 for details). These particular use cases need to validate that a set of variables loaded into the central Data object do indeed form a data frame:

• They are all present.
• They are all one dimensional variables of equal length.

This should be shared functionality, so a new method of Data.