Re-working strr_process_property

[dwachsmuth]

We probably want to use assignment by reference to get rid of extra columns, but may not want to modify the input table. So possibly put the column drop after the error checking, so that a copy has already been created once?

But maybe this doesn't follow, because it means that when a copy is created with row drops, that copy will be much larger because of all the superfluous rows?

I.e. need to check the memory implications of the different approaches.