numerous bugfixes. Requires Python >= 3.8

FIxed numerous bugs, including xarray API update

Requires Python 3.8+ as Python 3.7 library support and wheel availability is dwindling.

add sp3a read, additional nav3 header

also fix sp3 bug in reading microseconds

use Python hatanaka package, install unlzw3

• implement hatanaka using python hatanaka package, which provides Python version agnostic wheels
• install unlzw3 by default

read robustness

multiple fixes to handle RINEX oddities

Requires Python >= 3.7 due to enhanced type annotation

use src/ layout for robustness

also use entry_points instead of scripts

add sp3-d format

Thanks to @fmeynadier for adding sp3-d format to Georinex.

increased robustness

• add ability to overwrite .nc files with "overwrite=True"
• make Rinex3 files handle INRSS
• add more header parameters
• make SP3 reading much more robust
• tolerate blank lines starting rinex file (default up to 10)
• use "unlzw3" for pure Python .Z files
• general test / CI / coverage improvements

NAV: allow same-time entries, fix pandas >= 0.25, OBS2 wrong obs num OK

• NAV3: allow multiple entries at same time, instead of discarding them as previous versions did
• fix allows Pandas >= 0.25 to be used, there was a bug where pandas 0.25 starting outputting dtype('O') that broke xarray.to_netcdf()
• OBS2: allow more flexibility where the RINEX file specifies the wrong number of observation types. This isn't foolproof, but at least tries to allow these RINEX files with incorrect headers.

OBS2: max per-system SV is now 36

thanks @nmayorov #47

add .sp3 SP3-C file reading, interval header metadata

• add .sp3 SP3-C file reading
• add 'interval' metadata to xarray.Dataset output, if available in source data file.