Setup SGP4 Propagator with unit testing

[kyle-su1]

Purpose

Closes #366 .
Addresses points in parent pr. However, note that setup_sgp4() in positioning/tle.py is bugged, resulting in failure of test_get_sat_position() in python_test/positioning/test_sgp4_handler.py

New Changes

Moved gs/positioning/tle.py to gs/backend/positioning/tle.py
Added tests for functions in positioning/tle.py
Created sgp4_handler.py and setup sgp4 in accelerated mode
Made function to takes in a TLEData object and datetime to return the sgp4 progagator data as custom object SGP4Data
Added tests for functions in both tle.py and sgp4_handler.py in new positioning directory under python_test

Testing

tle.py functions:
Tested calculate_checksum with sample TLEs to ensure the correct checksum value is returned
Tested convert_decimal_point_assumed() returns expected decimal values. Note that this function fails on negative values, but I believe through research that TLEs will not have negative values that break the function so should work as intended in practice
Tested parse_tle_data() with sample TLEs with expected results
Tested parse_tle_data_invalid() to identify mismatched checksums

sgp4_handler functions:
Tested test_setup_sgp4() to ensure it returns valid Satrec object
Tested get_sat_position() with sample TLEs and datetimes. Currently works only on some TLEs. Returns eccentricity out of range error for the following sample TLE:

"CSS (MENGTIAN)\n"
"1 54216U 22143A 25308.16010129 .00032649 00000-0 38953-3 0 9993\n"
"2 54216 41.4668 251.0039 0006377 319.7576 40.2790 15.60341380158647"

Source of error should be setup_sgp4() in positioning/tle.py. Error occurs for TLEs where tle.eccentricity is low while tle.drag_term is high. See comments in testing files for specifics.

Outstanding Changes

Should create new pr for anyone that wants to tackle debugging

[github-actions]

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[Adityya-K]

Skimmed, will look at it once more and try to figure out the eccentricity issue!