Add methods to more easily convert GpxFile to/from string

[airbreather]

#13 had this comment saying, among other things:

The "quick-and-dirty" way to create a GPX model (that is, in cases where you will have lots of constant values to plug in, rather than just doing a small number of "new GpxLongitude(...)" calls) is likely going to look something like this, which lets you use double just fine since it's always safe to go the other way around:

and then showed an example of building a Gpx instance (now called GpxFile) "quick-and-dirty" from a string without hardly much ceremony at all.

This should probably actually be supported.