Add check for multiple primary flights on a single date

[ethanwhite]

There should only ever be one primary flight for a given date, but data errors can result in more than one (see #149). We should add a check for this that blocks the addition of multiple primary flights and throws an error, probably in combine_birds_site_year.py by creating storing the dates in a separate list in the for loop (in addition to adding them to eventdf) and checking the resulting list for duplicates.