Pyrinex

* added header mandatory fields sanity check
* simplified and sped up single-system RINEX 3 reading (pick single system from multi-system files).
* Multi-system RINEX 3 reading also available, but is significantly slower than single system reads due to non-uniform fields between satellite systems.
* BUGFIX: non-integer second timing crash

convenience function readrinex() handles OBS,NAV for RINEX 2 and 3.

added Python >= 3.5 type hinting

* greatly improve reading/conversion speed and memory usage by replacing Pandas NDPanel with xarray.Dataset
* much more extensive self-tests, tested against a wider variety of RINEX files.