column/index name hardcoded as strings - move to constants

[konstantinstadler]

the column.names / index.names are hardcoded as strings - move to constants.
There are not to many places where this is used, so it should be easy to change.

[konstantinstadler]

there is already a generic_names in constants - this is used in 2 different places:

1. for giving names to aggregated region/sectors
2. for checking iosystem and such

Either we separate this or find better naming to make the purpose clear. Currently best choice: remove the aggregation logic (this is quite specific and could just be a parameter) and use the generic_name for column names and such