Support short datasets #7
Description
Add exceptions for input data that has 2 or fewer YEARMARKERS (less than 2 full years recorded).
Current version returns an error for very short datasets due to problems with making model windows (in data_input function) and estimating groth rate for initial guesses (in run_model function).
These errors should be circumvented to support short data series.
If not possible, a more informative and user-friendly error message should be returned.