[BUG] Throw error when filling Int64 with negative weight

[andrzejnovak]

import numpy as np
import hist

h = hist.Hist.new.Reg(50, -5, 5).Int64().fill(np.random.randn(100))
h.fill(3, weight=-1)
h.plot();

Filling negative weights to the wrong storage type gives a silent error. This should either give a warning or just not be allowed at all.

[henryiii]

Fixed by #636, I think, but can add a test.