Library not returning datetime when valid datetime is available from battery-backed RTC on Ultimate GPS Featherwing

[TheOriginalJelibeli]

Issue is discussed in detail on the Adafruit forum here:
https://forums.adafruit.com/viewtopic.php?t=204255

The datetime can be parsed from the serial data returned by the GPS module, but as it is marked as invalid (V-marker?) due to having no GPS data, it is ignored by the library. The suggested improvement would be to examine the contents of the data whenever datetime is requested to see if there is a valid datetime, regardless of the validity marking.

Current workaround is to manually parse the serial data within the code, rather than relying on the library.

[tristan-huber]

Ran into this issue myself and wasted a few hours figuring out that the NMEA sentences still included the date info despite .datetime and .timestamp_utc not being available.

Sent a PR to fix this for RMC messages which is what I've been working with. LMK if you saw this with other sentence types which aren't addressed in #115.

There's definitely also some documentation improvements to be made around this since current Ultimate GPS v3 documentation mentions the onboard RTC but does not describe how to actually access the date info on the software side of things when there's no GPS fix.