ENH: improved type support for numpy arrays

[ntessore]

This change improves type support for numpy arrays in two ways:

1. Do not require numpy arrays to be aligned and contiguous. This fixes an issue with conversion from or to arrays of different int width.
2. Use numpy data types in C interface. Instead of treating numpy array data as the C types int64_t etc., the code now uses npy_int64 etc. to match the NPY_INT64 dtype descriptor.

A related change is to use order instead of nside for the UNIQ pixel indexing functions. This is a backward-incompatible change, but the functions are fairly new and not used much.