Refactor creating auxiliary, diagnostic, and derived fields

[baagaard-usgs]

Description

There is duplicate code for creating auxiliary, diagnostic, and derived fields. We can refactor the code to eliminate the duplication.

Desired Solution

1. The various implementations of the Physics object call addXXX for each subfield.
2. Move createXXXField() methods to the Physics abstract base class.
3. The various implementation of the Physics object also hold just one discretizations map. When adding a subfield, the physics object passes the discretization to the factory.
4. Factories would need to hold an array of (subfieldName, description, discretization).
5. Might be able to use template methods to reduce code duplication across IntegratorDomain, IntegratorInterface, and IntegratorBoundary.
6. Set default discretization via Pyre settings.