make electrical_neighbors.grid work with scenario path

[ulfmueller]

As we now are implementing the scenario path, the electrical neighbor task has to be slightly adjusted so it is well used also for the final year of the scenario path (in the current case for the year 2045). The task uses this function which has to be adjusted a bit like in the execute() function. So the 'elec' has to be replaced by 'base' and then it has to be assured that if a scenario path is executed, the prenetwork of the last year is used and if a eGon100RE-like scenario is used that one.

Moreover, (that is probably worth a new issue) for the upcoming tasks (after the pypsa-eur network is solved) like pypsaeur.electrical-neighbours-egon100, it is probably necessary to make these tasks also run for the last year of the scenario path...