fix(gridintersect): relax cell boundary checks with np.isclose

[wpbonelli]

• check np.isclose instead of exact equality in find_position_in_array() to allow for floating point error after round-trip coordinate transformation — if a coordinate passes the fuzzy check, clamp it to the cell boundary
• fix bug: GridIntersect's intersect method may throw IndexError when LineString starts outside model grid #2049

[dbrakenhoff]

Looks good to me! I think np.isclose defaults should be fine? Maybe @vincentpost has an example/test to add?

[wpbonelli]

@vincentpost I will merge so this can at least get some exercise in mf6 CI and maybe some user testing in the next couple weeks. If you have a test case please feel free to submit separately. thanks!

[vincentpost]

Sorry for responding to this so late, and for breaking my promise to implement the fix, so thanks @wpbonelli for doing this.

I have tried to design a test problem but I could not reproduce the error because it depends on numerical round off, so it's kind of random. But I have done a test with the model in which the problem originally occurred and I can confirm that the fix solves the issue #2049