Equations with One Unknown Considered NLAs

[RLee64]

Currently equations with a single unknown are considered NLA equations if the unknown is not by itself on either side of the equality. Below is a trivial example where x and z are defined, but the system is not capable of rearranging the equation to calculate y. Fixing this likely involves the use of a symbolic math library like SymEngine where we get it to try rearrange the types of equations we know it's capable of parsing.

[nickerso]

see also #232 and #654 for some (historical) context and discussion....or even way back to #130 for when we last put serious effort into trying to use SymEngine.