arrange functions into truly `numpy`-compatible structures

[v923z]

Functions can now be called in a numpy-compatible fashion, or, following circuitpython's convention, importing sub-modules, by appropriately setting the ULAB_NUMPY_COMPATIBILITY constant in

micropython-ulab/code/ulab.h

Line 31 in 5f7092c

#define ULAB_NUMPY_COMPATIBILITY (0)

However, some of the functions are actually scipy-related. In order to make porting existing code, ulab should be split into two sub-modules, each entirely compatible with existing numpy and scipy conventions. ulab could then be used as

from ulab import numpy as np
from ulab import scipy as spy


a = np.sin(...)
b = np.fft.fft(a)

c = spy.optimize(...)

etc.

[v923z]

Can be closed, since v.2.1 implements the numpy-compatible interface.