Update ChemTokenizer to split sentences as well

ChemTokenizer now returns a list of sentences, each of which is a list of tokens.
String to float utility function

Added a string to float utility function that is more accepting of different formats that just using the standard float function.
Move latex constants to separate file

Refactor lmtk.text so the long list of latex to unicode conversion constants are in a separate file.
Add a couple of lastnames
Small improvements to ChemTokenizer
Additional tests for ChemTokenizer