BUG: test load with pad failures

[jklenzing]

Describe the bug
Some of the load with pad tests do not work for cosmic_gps subsets. Some coords not present in all datasets, custom concat routine likely needed.

To Reproduce

1. Comment line 122 of cosmic_gps.py.
2. Run pytest
3. See results:

FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad0-inst_dict0] - ValueError: coordinate 'MSL_alt' not present in all datasets.
FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad0-inst_dict2] - ValueError: coordinate 'Lat' not present in all datasets.
FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad0-inst_dict5] - ValueError: coordinate 'caL1Snr' not present in all datasets.
FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad0-inst_dict6] - ValueError: cannot reindex or align along dimension 'RO' because of conflicting dimension sizes: {3688, 2860}
FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad1-inst_dict0] - ValueError: coordinate 'MSL_alt' not present in all datasets.
FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad1-inst_dict2] - ValueError: coordinate 'Lat' not present in all datasets.
FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad1-inst_dict5] - ValueError: coordinate 'caL1Snr' not present in all datasets.
FAILED pysatCDAAC/tests/test_instruments.py::TestInstruments::test_load_w_pad[pad1-inst_dict6] - ValueError: cannot reindex or align along dimension 'RO' because of conflicting dimension sizes: {3688, 2860}```