Prefer using open-source solvers wherever it makes sense

[tkelman]

This helps making it more accessible for anyone to run the code directly, wherever the notebooks aren't using any solver-specific functionality.

I could send a few PR's for this, but it'd be by editing ipynb files in nano since I don't actually use IJulia myself.

[mlubin]

Which notebooks don't do this?

[tkelman]

Whoops, I'll admit to not having read all of them. If it's only d610406, then that's not merged in master yet so false alarm.

[IainNZ]

False alarm, but its a good general point