[BUG] Raise an error when adding hists of different storage types.

[JohanWulff]

Describe the bug

Adding two hists of different storage types just results in having an empty hist without raising an error:

Steps to reproduce

import numpy as np
from hist import Hist
import hist
a_hist = hist.Hist(hist.axis.Regular(3, -1, 1), storage=hist.storage.Int64())
a_hist.sum()

0.0

b_hist = hist.Hist(hist.axis.Regular(3, -1, 1), storage=hist.storage.Double())
b_hist.fill(np.random.normal(size=1000))
b_hist.sum()

681.0

a_hist += b_hist
a_hist.sum()

0.0