Handle array access sensibly

[ibab]

Per-entry arrays are used quite heavily in ROOT files.
It would be nice if access to an array index in the columns argument would automatically lead to that array element being read out for each entry in the tree:

df = read_root('file.root', columns=['myarray[0]'])

The column should probably have a different name, like myarray_0 to avoid problems when writing it to disk.

[ibab]

Another idea:

When the given element (e.g. 'myarray[10]') doesn't exist for a row, root_pandas should enter a null value into the dataframe.

[ibab]

I think that it would be much nicer to just provide an option to flatten arrays when calling read_root.
We could make use of root_numpy.stretch for that.

[ibab]

I implemented the flattening idea.
This simple implementation should be enough for now.