HiGHS API: Add duals, slacks, reduced costs (#780)

[WPettersson]

This adds duals, slacks, and reduced costs to the highspy HiGHS interface.

Closes: #780

[WPettersson]

I've reverted this to a draft as I've realised it's not correct, there's at least a -1.0 multiplier missing for reduced costs for maximisation problems. I'll add some more testing of these parameters, and sort out any remaining issues.

[WPettersson]

Okay, now ready for review. I've also added another test for duals, slacks, reduced costs that involves a maximisation problem.

[pchtsp]

can you please apply the black linter as explained here: https://coin-or.github.io/pulp/develop/contribute.html#applying-the-black-linter-formatter
thanks!

[WPettersson]

Sorry, black has now been run, one line changed

[WPettersson]

I want to throw up a warning actually. I've been doing more testing, and I've found instances where HiGHs and CBC do not return the same shadow prices (constraint.pi values). I'm trying to narrow this down to a problem that can reasonably be solved by hand so I can work out which solver is right, but I wanted to let you know about a potential issue while I investigate further.

[WPettersson]

Okay, after a day of doing linear programming by hand, I'm confident that the solutions returned are all valid, the solvers are simply finding different shadow prices.