Improvments to expression parser with multiple brackets
Improved parsing of expressions; throws error if unknown function. Fixes #14

Many more expressions tested
Better import of Units in LEMS files, now reads offset & scale
More efficient handling of geenrated functions for runnable: if there's
nothing in update_state_variables, etc. don't add the method
Adding support for random() in LEMS

Updated README with latest NML2 example support