Draft-Antex parsing

[eulaliesa]

This draft PR introduces the following changes:

parse_atx(filepath: Path)
A parser for ANTEX .atx files as per the official IGS format (link). It extracts antenna PCO information per frequency and validity period into a structured DataFrame.

compute_pco_sat(...)
Computes the satellite Phase Center Offset (PCO) correction in ECEF frame, based on satellite position, receiver position, epoch and ATX metadata. It supports multiple frequencies and returns a matrix of corrections.

test_pco_sat() implements two synthetic test cases to validate the satellite frame to ECEF transformation and compare it against the output of the compute_pco_sat() function.

I have also added the functions ecef_2_satellite and compute_sun_ecef_position to /prx/util.py and the functions test_sun_pos and test_sat_frame to /prx/test/test_helpers