Extends the CF Checker, adds more Chapter 4 checks

- Moves DSPair to module scope
  DSPair is used by the compliance checkers and its structure is
  implicitly required in the Checker classes 'setup(self, ds)' methods.
  This commit moves DSPair to the module scope and explicitly defines two
  class attrs representing the struct elements.

- Adds tests for Chapter 2
- Adds static files for compliance checking
- Adds CF checks for Chapter 4
   - Coordinate vars for coordinate types
   - Latitude checks
- Adds 4.1 and 4.2 checks and tests
Merge pull request #1 from asascience-open/chap4

Extends the CF Checker, adds more Chapter 4 checks