Fixed insertion in CombinedKernel/CombinedFeatures

Since I didn't find the Combined*::insert() methods used somewhere and because append() is used instead, I decided to use array.push_back() in append and comment the function insert that it requires the index to be less than the number of objects.
Also added a couple of unit tests.
Plus, fixed bug that whenever a CCustomKernel was used in CCombinedKernel, the index of the features in a loop
shouldn't be incremented.