several functions to fix FIT files

[vlcvboyer]

several CLI functions to fix FIT files
-> smooth GPS trace (useful when bad weather corrupt GPS or for swimming activities)
-> combine several standalone activities into one multisport activity
-> change sport/subsport
...

[xmedeko]

@vlcvboyer My repo is a fork of the upstream dtcooper/python-fitparse just to do PR to the upstream. Unfortunately, my PR for writing FIT files has not been accepted. I do not plan to develop this feature and maintain a fork. Comment and vote for:

• Feature request: creating FIT file from any readable format (CSV, XML, JSON or any other) dtcooper/python-fitparse#8
• Add FitFileEncoder for writing FIT files dtcooper/python-fitparse#58

To gently push the fitparse authors to add FIT file saving, too.

Note, some actions may be done through https://www.fitfiletools.com

[xmedeko]

Update: I have decided do abandon FIT writing (and parsing, too) in pure Python since the FIT format is loosely specified and complex.
I recommend to make some FIT<-->JSON tool from the original FIT SDK (e.g. C#, Java, C++) and just call this tool from Python.