Rinexmod

_Under the hood_ improvements:
- add the `rinexmod` module (folder)
- add `setup.py` for assisted installation

:warning: inform the developer if you meet import troubles

- allows multiprocessing to fasten RINEX modifications (option `-mp`)
- new debug mode (option `-d`)
- ` -tol, --tolerant_file_period` option
the RINEX file period is tolerant and stick to the actual data content, but then can be odd (e.g. 07H, 14H...). A strict file period is applied per default (01H or 01D), being compatible with the IGS conventions
- `filename_data_source` (R, S, U) as modification keywords
- several miscellaneous bugs corrected

Add modularity in the behavior of rinexmod:

- a new module `rinexmod_api` has been created
- better RINEX3 handling
- several bug corrections
(a detailed help and tutorial will come in a future release ;) )

Misc. modifications in the options.

A bigger update is in preparation to allows to run rinexmod in an API mode.