Bug fix allowing decay descriptor check in combiners to work and warn about duplicates

This is a bug fix aiming to solve Analysis#22 (closed) and #260 The assert designed to catch bad decay descriptors was not working. Now throwing a runtime_error instead, which solves the issue Analysis#22 (closed). At the same time I added (an ugly implementation of) a warning flag about possible duplicates created by cc in the decay descriptors (#260). Possibly there is a more elegant way. I didn't want to throw an error, because there are examples where e.g. for [K+ K-]cc the two containers are associated to different inputs and thus the cc does not create duplication (a priori). Actually for cases like PID it is crucial that such combinations do exist, so can't throw an error.