allow multiple cuts on same parameter

[GarethCabournDavies]

Amend cuts module to allow multiple upper/lower cuts on the same parameter, also check and deal with duplicated cuts.

This is done by updating the cuts dictionaries to be keyed on parameter and the comparison function, rather than the parameter only.

Solves #4146

[spxiwh]

I'm struggling to review this, as I haven't seen this code before, and can't really tell what it is supposed to do or how it fixes the problem. Could we add a few simple units tests (or a single unittest code in /test) to illustrate and test what this module is doing?