BUG: verify handling of MultiIndex

[martinfleis]

It seems that when your GeoDataFrame is using MultiIindex, it does affect some parts of momepy and will likely break when using Graph. We should test and verify that it behaves as we want, both here and in libpysal. In some cases, raising may be the best option but if we can support it, it is better. I don't think that there's scope in supporting MultiIndex in Graph so it may need to be checked only in functions that do not use graphs.