Add conflict resolution methods to DataCollection.union()

[philsmt]

As discussed in #174, the current workaround assumes that processed files only ever modify all sources also present in the corresponding raw file. While true for the current facilty implementations , this assumption may be no longer be true when thinking towards custom processing supplied by users.

In the interest of sane storage policies, copying over all raw files should be discontinued too for facilty processing, and never be considered with users. Hence, it would be useful for DataCollection.union to support conflict resolution methods (such as "keep oldest", "keep last") to allow loading all these different locations into a cohesive DataCollection.