Upgrade legacy code for continuous models

[Mathieu Tanneau]

Current progress on upgrading legacy code to the new convention (i.e., rules, reductions and apply!-based syntax).

• Empty row (#13)
• Row singleton
• Forcing/Dominated row
• Fixed variable (#7)
• Empty column (#18)
• Free column singleton
• Dominated column

It would be nice to support QP problems as well.

[Joey Huchette]

I think I'll take a stab at empty row today.