MultiplicativeBasis does not use correct bounds

[wulfdewolf]

Describe the bug
MultiplicativeBasis does not have the bounds defined in its two sub-bases, this messes up evaluate_on_grid.
Also, get_equi_spaced_samples can not handle a list of None and numbers.

To Reproduce

import nemos as nmo
bspline1 = nmo.basis.BSplineEval(5, bounds=(0, 100))
bspline2 = nmo.basis.BSplineEval(5, bounds=(0, 100))
basis = bspline1 * bspline2
X, Y, Z = basis.evaluate_on_grid(100, 100)

print(basis.bounds)
print(X.min(), X.max())
print(Y.min(), Y.max())

Expected behavior
The passed bounds should be respected.

Suggested solution
We can add the following to MultiplicativeBasis:

@property
def bounds(self):
    b1 = self.basis1.bounds 
    b2 = self.basis2.bounds
    if b1 is None:
        b1 = [None]
    if b2 is None:
        b2 = [None] 
    return b1 + b2

Alternatively, but more work, the bounds could be a property of CompositeBasisMixin, and we can have a setter which sets all sub basis bounds.
That is, the user passes a list of bounds and the composite basis iterates over the list and assigns bounds to components.

[wulfdewolf]

For now, this can be easily by-passed by manually setting the bounds attribute on the MultiplicativeBasis object:

basis.bounds = (0, 100)