Exclude columns that are not one-dimensional

[KonstantinSchubert]

I think I found a much better solution than the previous one.

It should not perform any unnecessary copies.

I am unsure if I should inline the get_nonscalar_columns() function or if I should even put it into module scope.

I will close the other pull request because I think this here is a much better solution.

[ibab]

The whitespace here could be removed

[ibab]

This is great!
Can you also add a few tests to https://github.com/ibab/root_pandas/blob/master/tests/test.py that make sure that nonscalar columns are dropped and that there are no errors?

[KonstantinSchubert]

I hope this is fine?

[ibab]

Yes, looks good!
We should think about how to make this work better with the flatten feature, but this is fine for now.

[KonstantinSchubert]

I agree regarding the flatten feature. I agree that requiring the user to specify the columns he wants to flatten would be a nice solution would be (also to avoid unexpected consequences of the flattening). If the user attempts to flatten an column with higher-dimensional arrays, we can simple throw an error.