Disallow input of strict-inequality operators.

[rwe]

LPSolve doesn't support strict inequality, and it's extremely surprising to successfully add a constraint like lp.addConstraint({'x': 1}, '>', 0) and have it behave silently off-by-one. I believe it's a design defect in LPSolve that it supports that input format in its native format, and that defect shouldn't be introduced in a more modern interface.