Add function to convert inequality to less-than.
Add function to convert inequality to less-than.

This is not strictly correctly implemented. Currently, it takes an
expression of the form:

  left terms > right terms

and returns an expression:

  right terms < left terms

when it should more accurately be:

  right terms <= left terms

However, it is close enough for now. More accurate would be to invert
the coefficients, rather than the ordering.

alisdair/lpd3,alisdair/lpd3,alisdair/lpd3