hotfix(observations): enable using line_id or rno colummns as obsnames

[mnfienen]

There was an issue preventing the specification of line_id_column or rno_column as the obsname_column as well because the dataframe containing the obsname_column was being converted to object and, if it needed to still be numeric (e.g. line_id) then later the path dictionaries crashed. So, just flipping the info from those columns into a copy that is object. Tests included.

[aleaf]

Thanks!