preserve first instance of duplicated time

From what I've seen, where an SV has a duplicated time entry, the data for that time is duplicated as well. So I discard all but the first of those duplicated times steps for that SV.

Corrected NAV RINEX3 logic for irregular files

Made RINEX3 NAV reading for irregularly formatted files more robust.

Auto-read GZIP files as well as plaintext, transparently.

Python >= 3.6, flake8 and mypy type

As part of a general move to increase geoscience/geospace coding quality, we have moved to flake8 + mypy type validation, hence Python >= 3.6 is required.

Handle GPS RINEX 3 NAV edge case

...where FitInterval field is optional per RINEX 3.03 spec.

cleanup / bugfix

• fix bug reading RINEX 2 NAV
• cleanup / dedupe code
• add Galileo RINEX 3 NAV test

increase robustness of time stamp

Python < 3.7 handles microsecond precision, more than enough.

RINEX 3 non-integer second bugfix, increase robustness and speed

• 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

Add convenience readrinex() function, deprecate Py2.7

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

added Python >= 3.5 type hinting

Rinex 2/3 OBS/NAV reading for one or many satellites

• 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.

v1.0.0

pass selftest