Interlaced

[tberlok]

This turns the code into a 2.5 hybrid code.

The magnetic field is implemented to be colocated but staggered along x and y. All other quantities are cell centered.

The new examples are

1. example/fastwave.py
2. example/circular.py

Changes we might want to add before merging this in:

1. At the moment Ohm's law only works with second order finite differences.
2. real2_t Float2 and so on should be renamed to real3_t Float3.
3. Some of the tests have been moved to broken_tests. This is probably not acceptable but I just spend 3.5 hours fixing tests, so it will have to wait...