Allow dtypes with byteorders via lists. Fixes #434

[tamasgal]

This patch fixes the byte order interpretation of dtypes which are passed as lists. The current implementation ignores them and falls back to big endian in case of the ROOT data (fromdtype) and native for the Python side (todtype).

Note that the workaround is to pass numpy.dtype instances, which works fine, so this patch is more like cosmetics, but can potentially prevent some confusion in future.

[jpivarski]

I see—so if an explicit byte order is passed, we take it seriously, even if it's in a record dtype. That sounds like a missing case for a previously intended behavior.

When you give an explicit "merge it now" and all tests pass, I'll merge it.

[tamasgal]

Great, yes please merge then 😉