Refactor array-loading methods, add tests

[mwtoews]

This refactors the newly added intersection_v and nearest_v functions, and adds some tests.

• Allow functions to take lists as inputs, using NumPy's atleast_1d and atleast_2d functions to sanitise inputs for ctypes
• Replace assert checks with more verbose checks that raise ValueError, checking dimensions, shape and strides
• Change the function signature of nearest_v to enforce keyword-only from num_results; this function is new, so I don't anticipate this change will be disruptive. Note that nearest does not enforce keyword-only from num_results, but this could change in a future release through a deprecation process.
• Add some basic tests