Add LaTeX splitter
P.S. My twitter is also `janldeboer`  :)
This PR is adding a LatexTextSplitter, closing #1731
It's based on the MarkdownTextSplitter, but the separators are adjusted.
I also added an entry for it to the according `test_examples.ipynb`

One issue I have, is that I can't run the complete notebook without an error in a later part, I'll open up a new issue for that.
