Add meaningful error message for 1-stratum data-sets

[AdamCSmithCWS]

Currently, datasets with only 1 stratum (species with very small ranges and coarse grained stratifications such as BCRs), throw an obscure error in the run_model() function. The error references a dimension mismatch that relates to a multi-dimensional array that should have a dimension defined by the number of strata.

To do:

1. Short-term: add a meaningful error message. include suggestions to try a finer grained stratification (latlong).
2. Long-term: adjust the code and models so that the models can be fit using only a single stratum.