Replace `SourceField` for `UniformBackgroundField` #193

Job	Run time
Ex1 (3.8, ubuntu-latest, tests/test_dcip.py)	36m 38s
Ex1 (3.8, ubuntu-latest, tests/test_em.py)	2h 57m 44s
Ex1 (3.8, ubuntu-latest, tests/test_gpr.py tests/test_seismic.py)	4h 5m 51s
Ex1 (3.8, ubuntu-latest, tests/test_inversion.py)	35m 35s
Ex1 (3.8, ubuntu-latest, tests/test_gravity.py tests/test_mag.py)	35m 39s
Ex1 (3.10, ubuntu-latest, tests/test_dcip.py)	1m 29s
Ex1 (3.10, ubuntu-latest, tests/test_em.py)	2h 57m 24s
Ex1 (3.10, ubuntu-latest, tests/test_gpr.py tests/test_seismic.py)	3h 31m 32s
Ex1 (3.10, ubuntu-latest, tests/test_inversion.py)	1m 11s
Ex1 (3.10, ubuntu-latest, tests/test_gravity.py tests/test_mag.py)	36m 14s
	15h 59m 17s

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