Improvements and Fixes for go-gpx

[inode64]

Title: Improvements and Fixes for go-gpx

Description:

This pull request includes a set of changes aimed at improving the functionality and handling of the go-gpx library:

• Included support for Garmin schemas.
• Fixed issue regarding the definition of email.
• Made extensions optional.
• Attached are more significant tracks for testing.
• Added support for GPX 1.0 management due to missing tags like name, description, author, email, and others.
• Modified the GPX structure, given an issue in GO not correctly managing XML namespaces prefixes. Initially, this problem drove me crazy, so I even created a sample to make it easier to visualize: https://go.dev/play/p/0M_tCWNJ-0w. Here is the solution to the problem: https://go.dev/play/p/4MpYbo33-gc. There's an open ticket regarding this issue at encoding/xml: support for XML namespace prefixes golang/go#9519.
• Hence, I've removed the use of XMLAttrs and have searched for all Garmin extensions amongst my 2000 tracks. These have also been useful to ensure that no information is lost when using this library.
• I've had to change the date format from time.Time to string as it occasionally wasn't present.

These modifications should improve the overall functionality and reliability of the go-gpx library, especially regarding the management of Garmin schemas and GPX 1.0.

[twpayne]

Thank you for your work here. This is a very large PR that will take some time for me to understand. In the future, please can you follow Dave Cheney's talk, then code approach.

[inode64]

okay,
I have tried to separate the improvements and bugs in different commits
If you want, create different pull requests for each change.

[twpayne]

Please can you separate this into multiple PRs, with each PR addressing one aspect.

[twpayne]

Gentle ping. I'm keen to see these contributions in go-gpx.