Refactoring ipopt solver

[MaxvdKolk]

@artofscience Could you take a look at the proposed changes?

Unfortunately not much progress on reducing the overlap between the IP solver variants. However, I did reoder the code a little:

• Some more verbose variables names to clarify the main optimisation loop
• Moved the variables lists (self.w) to a class carrying the variables, such that we can access the variables by names, for instance self.w[0] becomes self.w.x. This might make the equations a bit more readable, as it directly states the variable names rather then list indices.

I have not (yet) checked if this might have implications for performance though, but I will still try to do that. The tests still seem to pass, so I assume everything is still working as intended, but we might want to double check before merging this request.

[artofscience]

@MaxvdKolk can we merge this?