prune team numbers from tracks on export

[caver456]

this would address the sartopo feature request to 'only send assignment letters, not assignment numbers, to GPS devices'; is it a good idea? could be problematic if done accidentally

[caver456]

Added the code to do this in a new commit, but some of the caveats bring up the question of whether this is a good idea; it's easy enough to disable this if needed, either via hardcode or via GUI setting:

                #  PITFALLS of this method:
                #  - This represents a difference in generated output between Export-GPSIO and Export-GPX.
                #      So, if one GPS device is written using Export-GPSIO, but another GPS device
                #      is written using Export-GPX, they will see different track names in the field,
                #      and the goal of reducing confusion will be negated.
                #  - There could still be unintentionally modified names, e.g. 'Track 3' would be changed to 'Track'.
                #  - This would not work if the second word contains letters (e.g. team 103b)