readthedocs build does not allow for escaping underscores in `:math:` mode

[glemieux]

While checking the automated readthedocs build for #61, I was noticing that the t_soisno_sl name was not displaying correctly. Adding a backslash escape character does not seem to allow for the underscores to be evaluated correctly. I can get this method to display correctly with my local sphinx environment build however, so we perhaps matching the configuration will help. I'm not sure if there is any easy way to export a local conda environment to match a readthedocs config file.