Document NavierStokesSolver.

 * Add brief descriptions to member functions of NavierStokesSolver.
 * Write createVecs as one function templated over dim.
Add unit tests for CartesianMesh.

 * Use the Google C++ Testing Framework.
 * Add test cases for uniform and stretched meshes with one subdomain.
 * Add member function initialize to CartesianMesh.
 * Edit makefile to accommodate changes.