Implemented models

Various models that use the chombo-discharge solvers and functionality have been implemented and are shipped with chombo-discharge. These models implement TimeStepper for advancing various types of equations, and have been set up both for unit testing and as building blocks for more complex applications. The models reside in $DICHARGE_HOME/Physics and are supplemented with Python tools for quickly setting up new applications that use the same code. In general, users are encouraged to modify these models and tailor them for their own applications.