tsdb.format() handles datetime.datetime well but not datetime.date

[goodmami]

The tsdb.format() function, called when writing a TSDB database, will convert datetime.datetime objects into the TSDB-standard format (i.e., DD-MM-YYYY) but not datetime.date objects. The current situation isn't terrible, as it will just cast them to a string, which is YYYY-MM-DD and the tsdb module can read those in just fine, but it could be better.