Refactored TCXRead initializer into load_file and parse.

[dblock]

After trying to use TCXRead in dblock/strava-ruby-client@85b501d ...

Refactored the implementation to support loading raw data (not always a file).

Instead of TCXRead.new, use load_file and parse.

• Preserved backwards compatibility, but I think .new should never be used and could go away, could just delete that and do a major version bump.
• It's not possible to do everything inside new, because you can have a file path or raw data as a String, so all the libraries like JSON support load and parse.
• Happy to rename load_file to load if you think it's more idiomatic.
• Got rid of Gemfile.lock because this is a library.
• Split up tests to avoid unnecessary setup.

[firefly-cpp]

Dear @dblock,

Thank you for reporting the issue and providing a fix. The next logical step is to release a new version (I will do it tonight). Feel free to add additional functionalities. I recall we didn’t support all features from the TCX files.