ContinousEnergyTable secondary particle block retrieval does not range check the index

[whaeck]

Secondary particle blocks like SIGH, etc. do not perform an index range check in DEBUG mode contrary to what the doxygen documentation tells us. Probably an oversight on my part when I refactored that part of the code.

In addition, we should also provide access functions for each of the vectors with these blocks (this would make copying over the data easier - no need to put everything into a vector yourself).