fix: axis passthrough

[henryiii]

Followup to #977. Uses the new axes if one is provided.

I wonder if we should allow axis + other args, if someone wants to use a new axis with a custom mapping?

We'll have to work out the correct mapping for flow bins for the four cases.

[henryiii]

What would you expect to happen if you rebin an axis that doesn't have flow? Specifically:

h = bh.Histogram(bh.axis.Regular(5, 0, 5, underflow=False, overflow=False))
h.fill([-1, 1.1, 2.2, 3.3, 4.4, 5.5])
hs = h[bh.rebin(edges=[0, 3, 5.0])]

Should hs.axis[0] have flow bins?

[henryiii]

Actually, I think it should match rebin(2), which matches the original. :)