Move and edit conditioning material

- Adds a heuristic discussion of conditional densities to the probability
  introduction
- Moves conditional probability material to discussion of probability
  spaces and discussion of independent sigma fields to the information set
  section
- Moves outline of conditional expectation to the section on expectations
Edit probability text

Makes a few minor corrections to the text of the probability material.
Add 'list of tables' and 'list of figures' to probability document

We'll wait for other documents to have tables and figures before adding
similar lists to them.
Edit expectation material

Consolodate the material on expectations and conditional expectation, and
start a first draft of actual text for that section.