Clean up notebooks

Address #82

Update description in the introduction to reflect the correct bases
(i.e. not continuous, but the discrete). Include $n$ for n_states in
the summation.

Clean up the Legendre poly notebook by improving the dialogue.
Fix types in notebooks

Address #82
Switch to keyword arguments for more of the API

Address #82

Keyword arguments mean that the order of arguments does not have to be
remembered.