Revisit index re-assignment when writing LAS file

[dcslagel]

The merged pull-request #3 re-assigns the index column based on column name if a LAS 2.0 specified index column is available when creating a las object from Dlisio. Re-visit this implementation and look at first calling the index assigned to the frame natively with the index=curves[frame.index].

Maybe it makes sense to make this an optional feature, defaulting to off.