Fix dist_approx with "geosphere" method and close points

[tovogt]

Changes proposed in this PR:

• Fix the dist_approx util function when used with method="geosphere" and log=True.
• This only affects very small input values, i.e. computing the distance between points that are very close. I added a unit test for that case. I also added a test case where points are identical because I think that this is a kind of sanity check that should be included.

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[emanuel-schmid]

Looks good to me. Obviously fixes a bug.