Signed Analytic error plus documentation

[ryanmrichard]

Is this pull request associated with an issue(s)?
No.

Description
This PR adds:

• some documentation on integrals (I had intended to work out error models there, but then got distracted; I'll return to the documentation in a future PR).
• Changes how the analytic error model works so that it predicts a signed error.
• Creates a TaylorFactory (I will generalize tomorrow) to create a single source of truth for the process of propagating UQ settings (such as TaylorModel's truncation order, and eventually TAffine's threshold) and the UQ type to use.

TODOs

• Generalize TaylorFactory to UQFactory