Error on modify-then-query

[odow]

If you solve a problem, modify it, then query a solution, the result is undefined behavior.

We've had a few bites at this:

• Querying after modifying is undefined behaviour #2076
• Issues when fixing variable value using CPLEX #2400
• [docs] Clarify modify-then-query warning #2401

The docs currently say

but the warning is buried in the documentation and it keeps tripping people up.

I would propose we add a flag to check this at the JuMP level and throw OptimizeNotCalled. Asking every low-level solver to do so seems like too much work, and some solvers are fine with it.

The other option is to do it at the CachingOptimizer level.