Change ts_map default MOC refinement strategy

[jdbuhler]

To address issue #465 , this patch changes the default pruning strategy used by MOC ts_map from top-k to containment-region-based. The top-k strategy is prone to implementation-dependent behavior when many pixels have the same likelihood (usually 0). The first test case for MOC ts_map has been updated to reflect the new default.

I incidentally fixed a couple of syntax warnings in fast_norm_fit.py caused by backslashes in docstrings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.18%. Comparing base (4911568) to head (42e7b71).

Files with missing lines	Coverage Δ
cosipy/ts_map/fast_norm_fit.py	49.50% <ø> (ø)
cosipy/ts_map/moc_ts_fit.py	97.89% <100.00%> (ø)

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