Remove noexcept from ParallelIndexSet::operator[]
While it does not throw an exception, it does call std::vector::operator[] that has no noexcept specifier and therefore might throw.
This reverts parts of MR !236 (merged) and is inspired by @carsten.graeser's comment. I agree with him that we should be careful with noexcept specifiers. Not sure whether we should revert the added specifiers also in timer.hh. I did not do that as they either call C functions or noexcept C++ ones.