Add support for Chemical Reactions and more symbols

[ksinkar]

• The additional symbols are very helpful in notebooks for Astrophyics

Supported symbols by mathjax: https://docs.mathjax.org/en/latest/input/tex/macros/index.html#environments

[shreve]

Would you be able to add or update a test to demonstrate the usage of these packages? I think a place that makes sense would be tests/files/notebook1.ipynb because it's a demonstration of library that supports outputting LaTeX equations.

[ksinkar]

@shreve I have added another file tests/files/notebook6_physics_chemistry.ipynb. The reason I did not touch tests/files/notebook1.ipynb is because it is generating code from python functions, rather than converting markdown code from a notebook. I hope this works. I tried running the tests as specified in README.md but did not quite understood the output. Please let me know if the new test code absolutely needs to be in tests/files/notebook1.ipynb, I shall do that. But IMHO we should keep this in a separate file.