Rewrote datetime parsing code to use isodate

According to the specification of GPX, the only format for datetime values is xsd:dateTime, which is an ISO 8601 date format.  Things like ISO8601 "without T" which is included in your tests aren't actually valid for GPX files at all... so I have removed this.  If there's a piece of software that produces files in this way, you may want to look at using an [attitude readjustment tool](https://en.wikipedia.org/wiki/Hammer) on the developer of that software. :)

I'm using the `isodate` library as it is more fully featured than the existing code was, supporting such things as being TZ-aware, and not trying to strip out timezone information that is there.  I've also removed the unused `parse_time` function that was used in unit testing, but never actually used when parsing a GPX file.

There's still some more work to be done, like making the datetime parsing tests actually check the output of that function, instead of just checking if the function failed to execute.  But given it's using a third-party library that has unit tests, this may be entirely redundant.