making rfta test run faster and fixing bug in rfta

[odedstein]

This PR makes the RFTA tests significantly faster, and it also fixes a subtle bug in the RFTA function. It also changes an RFTA default argument. There is also now an option in the RFTA function to force CPU rasterization.

[sgsellan]

From the action:

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FAIL: test_noop (test.test_reach_for_the_arcs.TestReachForTheArcs)
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Traceback (most recent call last):
  File "/Users/runner/work/gpytoolbox/gpytoolbox/test/test_reach_for_the_arcs.py", line 68, in test_noop
    self.assertTrue(h < 0.1)
AssertionError: False is not true

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