Apply nan/0 mask to output of goldstein filter

[scottstanie]

Closes #261

For a current input with rotated phase (e.g. a geocoded interferogram):

The edge blocks leave slightly non-zero values (here is using np.ma.masked_equal(phase, 0)):

The result of this PR:

[scottstanie]

spurt appears to be hanging... at first it was in test_unwrap.py before i made the test size smaller, but the last time it was in tests/test_workflows_displacement.py::test_displacement_run_extra_reference_date[spurt] (never appears in the log). Unsure how to debug, since the tests run fine locally on linux/mac