Fixed a ConcurrentModificationException bug. Made a copy of a set before
iterating over it, so that values can be inserted into the set.
Merge branch 'bugfix/mappingProcessorIteration' of github.com:ontop/ontop into feature/viewKeys

This bugfix must be merged into the viewKeys branch, as otherwise the branch cannot be used on the mappings where it is needed
Added foreign key reading to the view key reader manual "fake" keys
Prints out all foreign keys to the console. Added a view_keys for the
testing.