Ashwin/z tracking

[betaBison]

• allows rinex_obs to load all possible observation codes
• z-tracking observations may have empty carrier_phase and raw_doppler_hz rows
• fixes #139

[betaBison]

example navdata result from the unit test data with z-tracking values. Note that carrier_phase and raw_doppler_hz rows are empty for all the 1W observations.

z_tracking.csv

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (35445af) 99.63% compared to head (f5e7637) 99.63%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v0.2.3 #145 +/- ## ======================================= Coverage 99.63% 99.63% ======================================= Files 21 21 Lines 3545 3553 +8 ======================================= + Hits 3532 3540 +8 Misses 13 13 ```

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