Equations are not always formatted properly

[rabernat]

We need to figure out why and fix it.

Example: https://m2lines.github.io/L96_demo/notebooks/presentation-model-setup.html

[breichl]

It looks like this can be fixed by adding the "dollarmath" extension into the _config.yml file. I will follow with a commit including this addition.

[breichl]

PR #48 fixes this issue in many notebooks, but (for example) there are still some equation issues in Neural_network_for_Lorenz96.ipynb. That specific issue might be linked to the headers issue #38

[breichl]

This issue was solved with PR #48, other issues are separate.