More SINEX convenience functions for GNSS module

[adeane-ga]

Description

More read functions have been added to the gnss.py module to account for more block types in SINEX files. The writeSINEX() function has also been updated to reflect this. This work is related to the update of the GDA2020 National Adjustment workflow to account for the NGCA solutions that come from AUSPOS-v3 with SINEX files in ITRF2020.

Functions Added:

read_sinex_file_reference_block()
read_sinex_input_acknowledgments_block()
read_sinex_solution_statistics_block()
read_sinex_site_receiver_block()
read_sinex_site_antenna_block()
read_sinex_site_gps_phase_center_block()
read_sinex_site_eccentricity_block()
read_sinex_solution_apriori_block()
read_sinex_solution_matrix_apriori_block()
sinex2dataframe_solution_apriori()
dataframe2sinex_solution_apriori()

[adeane-ga]

No urgency for this merge. I'm still working on updates to the gda2020 workflow which may inform more changes to this. But, what is contained here, should be the bulk of it.

[harry093]

Excellent work, mate!