I do agree, however I would prefer not to have two separate interfaces.
This may get confusing if we were to have two separate interface as I will be developing another branch after this PR has been merged which will split the functionality of this slices method, to a "slices_to" and "slices_over" methods (names not yet decided) - see [link](https://groups.google.com/forum/#!searchin/scitools-iris-dev/friendly/scitools-iris-dev/mLMjmzTlCXs/aF3Ph96HPIMJ).

If we would split the functionality here, then we would have four methods for slicing, which is messy.