Update/improve macOS install

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https://practicalprovenance.wordpress.com/2020/12/08/installing-provtoolbox-on-macos/ documents installation requirements on macOS, including OpenJDK and Graphviz which most students do not have installed.

While installing OpenJDK has quite a few options on macOS, this can be a bit bewildering, so I used https://adoptopenjdk.net/ - both their OpenJDK JRE 11 and JDK worked well.

For Graphviz it is unfortunately a bit more messy, https://graphviz.org/download/#mac just recommends HomeBrew or MacPorts. I found HomeBrew no longer working on my machine as I did not have the bleeding edge macOS, so I also documented how to install it with Conda.

I've added this guide to https://github.com/lucmoreau/ProvToolbox/wiki/Installation as our users could not get the 0.7.3 DMGs to work - perhaps a Mac user could investigate further and look at ways to bundle Graphviz and OpenJDK within the PKG, or perhaps better, add provtoolbox to HomeBrew.