Make check for inputGeneratorFile stricter

Changes introduced

Check for runArgs.inputGeneratorFile was based on assigning values to a dictionary, which would throw an exception (and thus not be checked) if the user would have sth like

runArgs.inputGeneratorFile = someNonExistentValue + "string"

see !2898 (merged)

Changing this to just throw an error whenever runArgs.inputGeneratorFile is present in the jO (even if it is commented out).

Tests

Issues resolved

Closes #210 (closed)