Add tests

[willirath]

(From #3 (review).)

• test the XoakAccessor index property
• test non-transformed coordinate values (perhaps with other data than geo coordinates)
• test NN lookup tolerance
• check that the order of coordinate labels in the returned selection is preserved (I checked this locally simply by inspecting the results and it works)
• shallow test for the DataArray.xoak accessor

[benbovy]

Closed in #10, #30.