add check for cutoff for graph traversal for prospective databases

[muelleram]

The current approach traverses the original database to stop at a certain defined cut-off. It may happen that in a prospective database the production technologies are so different that processes above the cutoff that have a large impact are excluded or processes below the cutoff are included.

[muelleram]

This could be implemented by calculating a static lca with every time-specific database and then, during the priority-first graph-traversal, inform the cut-off algorithm with most conservative score of each visited node from the set of scores with the different time-specific background databases.

As long as we do not have temporal information in the background, this is not so relevant as we can set the cut-off to very low to make sure we continue the traversal throughout the comparatively short foreground systems to reach the intersection with the background dbs, where we are currently stopping anyways.

[jakobsarthur]

TODO: add warning to change baseyear database.