allow posparam fitting less than one day of data

[sbailey]

Allow fit_posparams to work on a single day of data. Previously this fithandler.py code wouldn't process any data if len(datum_dates)==1:

    for j in range(1, len(datum_dates)):
        start_date = datum_dates[j - 1]
        ...

The solution here is to catch that as a special case ahead of time, leaving the existing _define_cases behavior unchanged for the original use case of grouping many moves over many nights.