remove numpy build dependency

[Max Bachmann]

numpy is currently required as build time dependency for cdist. This leads to a lot of issues, which would be fairly simple to avoid by using numpy only in Python:

def cdist(...)
    import numpy as np
   
    result = cdist_cpp(...)
    return np.asarray(result)

as long as the result of cdist_cpp follows the buffer protocol, this wouldn't require a copy of the matrix. Since cdist is run with many elements the overhead of this Python wrapper becomes irrelevant. In addition this makes this change backwards compatible.