Starting values for constant expression

[blegat]

See https://discourse.julialang.org/t/wram-start-of-hermitianpsdcone-in-jump/105491/10?u=blegat

[odow]

So I implemented this locally, but I don't know about it.

Even if we fix the constant case, the example will still fail for imag.(x) because the lower-triangle is -x.

It also breaks our current rule of "iff the expression is convertible to a variable."

[blegat]

For the lower triangle, we could set the opposite of the starting value. It then set twice the same value to the variable. We could allow starting value if we have at most one variable and possible a nonzero constant and non-one coefficient

[odow]

So now our scope is starting to widen... How about we just add the set_start_value function from Discourse into the docs for HermitianPSD?

[odow]

See #3564