Add HiGHS as a solver

[trevorb1]

Description

In this PR I add functionality to process HiGHS solution files. Specifically, I have:

• Added a new ReadHighs class as a read strategy
• Added tests for the ReadHighs class
• Documented how to use HiGHS (both from the command line and python)

A couple other notes:

• As described in the original issue, there are a few solution formats to choose from. I went with the one I called HiGHS Format in the ticket (solution style 1 in HiGHS docs). Motivation is that it only requires one file as an argument and has dual values in this file for if we eventually want to extract out these values.
• Going with solution style 1 requires us to pass in an option file to the solver from the command line. A little cumbersome, but not a big deal (and this is documented!)
• For processing a HiGHS solution into a wide format dataframe for otoole, I believe we can do most of the processing through highspy, but I chose not to. It is probably a little more efficient to use their package for bulk processing, but then that adds a dependency to otoole
• Getting the CLI of HiGHS cant be done through a package manager - instead requires installing a binary and setting up environment variables. Therefore, I also provide an example on how to just use highspy and otoole's python API to process a solution file.
• I strip out zeros from the solution file to be consistent with how we process CPLEX files; however, this is also an open issue! (see issue [Bug]: CPLEX results processing does not return zero values #214)

Issue Ticket Number

Closes #218

Documentation

I have updated the following pages:

• Core Functionality with notes on otoole being able to process HiGHS now
• Examples:
  • Provide instructions on how to install HiGHS CLI
  • Provide instructions on how to install highspy
  • Provide example on how to process HiGHS from command line
  • Provide example on how to process HiGHS from python

Instructions for setting up HiGHS to use from the command line may need to be improved a bit. For example, I had to add HiGHS executable to my .bashrc via the following command, that I did not include:

alias highs="/opt/highs/bin/./highs"
export PATH=$PATH:"/opt/highs/bin/"

I don't think the instructions should necessarily turn into a "how to setup solvers" guide; as there are better resources than otoole for that. However, it can be kinda tedious getting everything working. Additionally, I kinda glossed over install for Windows as I havent done that!

[willu47]

I've read through the code, but haven't run it (or followed the instructions in the documents to test implementation - which I'd ideally like to do).

Looks great!

Could you fix the conflicts @trevorb1, and then I'll try to follow through the instructions to test the functionality as you've written it, just to sense check on my own laptop before we merge and release!

[trevorb1]

Thanks for reviewing, @willu47! :)

I updated this PR to address conflicts. I updated the docs to (hopefully) make them clearer.

Also, I was doing a bit of testing with HiGHS and I guess consistent reported columns are not guaranteed. For example, the Type column only seems to get written out if different types of result variables are present (see here). I couldn't find docs on what columns to expect, so we may run into issues in the future when a column name is added or removed. This conditional should catch most of these issues though, I believe.

If anything is unclear in this PR, please just let me know!