Ensure FIT-to-TCX handles all test files from python-fitparse correctly

[dlenski]

This PR adds dtcooper/python-fitparse as a submodule so we can test against all of its test files. Many of these were not correctly handled by FIT-to-TCX previously.

Then it fixes the resulting issues:

• Handle optional/missing fields better
• Handle files which only have local/relative timestamps
• Readable error for FIT files which contain no activity session message

Also, attempting to parse more of the test files explores more of the branches, and thus increases coverage.

[coveralls]

Coverage increased (+1.7%) to 90.452% when pulling 6cc441b46687f19a3c22d20719247c6843f620b9 on dlenski:master into 8988a174d671095182c950066b489597befd1447 on Tigge:main.