Convert vector of vectors or array of pointer of pointers to numpy array and back #7

To enable pickling, we basically need to convert proj_vecs in Cython <-> a numpy array or numpy memoryview?I think it would just be a double for loop or something? This is required to enable pickling, since we need to tell the class how to convert things into a readable Python object, and then also how to go from a readable Python object to low-level Cython code.

We have to decide whether it is easier to convert vector[float] -> numpy array of floats or
float * -> numpy array of floats.

• Converting numpy array -> pointer: https://stackoverflow.com/questions/10718699/convert-numpy-array-to-cython-pointer
• Convert vector -> numpy array: https://stackoverflow.com/questions/59666307/convert-c-vector-to-numpy-array-in-cython-without-copying