Quantecon-book-theme

This release adds `textmacros` package as an extension when using Mathjax v3.

This release corrects the homepage link in all the pages of the project, specifically for cases when the pages are in subfolders.

This release checks for the presence of the `_notebooks` directory to show the download button.

This release has a few enhancements:
- center aligning figures, supporting output_scroll tag
- countering pyLDAvis package stylesheet overwriting

This release has added codecov token explicitlt in codecov.yml , as codecov was complaining about it.
It also has following two features:
- Checking for mathjax version and using the config specified in `layout.html` when mathjax is of version 3. Users can use `mathjax_config` variable in `_config.yml` for version 2.
- Left-hand side menubar can now be made persistent across web pages using a config variable.

This release has fixes for the downlodPDFModal functionality and styling.