[ENH] Use scipy.sparse for bsplines and tensors

[dswah]

Instead of homegrown tensor-product.
This way we avoid materializing the dense model matrix for our model matrices

B-Splines

https://docs.scipy.org/doc/scipy/reference/generated/scipy.interpolate.BSpline.html

Tensors

scipy.sparse.kron:
https://docs.scipy.org/doc/scipy/reference/generated/scipy.sparse.kron.html

to replace:

pyGAM/pygam/utils.py

Line 897 in d9789ce

def tensor_product(a, b, reshape=True):