Reorganize linear regression material

This commit moves around some of the linear regression material;
- Asymptotic statistical properties are moved to the asymptotics document
- Algebraic properties and multiple testing are moved to the linear
  regression document
- Move material about normal r.v.s and quadratif forms to the probability
  introduction
- Some edits are made to the finite sample inference and estimtion documents.

reorg
Reorganize finite-sample material

This commit moves the finite sample material around and prettys up its
source code.