propositional logic <=> propositional calculus

Adds a section explaining this. Also gives latin origin of "calculus".
Fixes whitespace in title
Fixes equation reference
Slightly improved ODE handwaving ...
Improves position equation derivation
Comments integrating factors section.

Underdeveloped; doesn't fit in to structure of my notes. I'll re-add
this later.
Gives picture for def'n of function
Improves reciprocals/deriving identities sections
Removes label from repeat of eq:pythtrig
Improves punctuation; fixes reference.
Removes extra whitespace
Moves examples to under defns; moves unit circle.

Unit circle: [H] -> [h].

Examples referencing right triangle figure are moved to underneath sine,
cosine, and tangent definitions.
Removes `notes.out` compiled source