Fix floating-point precision artifacts in longitude normalization

[mortenwh]

When normalizing longitudes to ±180°, float arithmetic produced representation artifacts (e.g. '41.76' → '41.75999999999999'). Fix by rounding back to the original number of decimal places.

Also handles numpy.float64 inputs by converting to str before inspecting decimal places.

Summary: closes #358

Related issue: #358

Suggested reviewer(s):

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