Fix example in Shell 6

We are now looking for the least common species so the intermediate
files should be called least_common_species
Add class lecture on expressions, etc.
Add class lecture on modules
Add class lecture on state in IDEs
Add class lecture on using help
Add class lecture on shell
Expand ipynb on strings

Adds information on the string module and string methods.
Add link to strings IPython notebook

And cleanup a few links to notebooks in the old repository.