Assess and pick MATLAB version for HyEQ v3.1 and update code to take advantage of worthwhile features.

[pwintz]

This page has a list of major features: https://www.mathworks.com/matlabcentral/answers/492752-table-of-matlab-release-features#answer_402944

Version R2017a seems like a minimum version because it includes

• R2016a Live Scripts
• R2016b local functions in scripts, string type
• R2017a double-quoted strings, expanded object properties validation

Later useful updates are listed below, but I think that 2019b might be too recent for some users.

• R2019b arguments block for function input validation
• R2020a default file encoding is UTF-8
• R2021b improvements to code editor (block editing, automatic suggestions/completions, refactoring, etc.);
• R2022b dictionary data type

[pwintz]

I've decided that we will officially support R2017a and later, but will generally try to not make changes that break compatibility for older users. One exception will be updating the Simulink models to a newer version of MATLAB, namely R2017, which will make them unopenable on older versions of MATLAB.

[pwintz]

Let's just keep the code as-is, but stop testing all of the older versions. We'll run the tests on 2014b, and then on 2019a-current.