Electrokinetics updates

[kevinstratford]

• [x] Field implementation
• [x] I/O
• [x] Kernels
• [x] PETSc solver
• [x] Optional PETSc tests
• [x] Documentation
• [x] Re-integrate reasonable tests

[kevinstratford]

Note on some of the tests.

The second Gouy-Chapman test has been removed, as it is a copy of the first one.

Other tests need to be shortened.

[kevinstratford]

Some residual issues, but progress in made.