Replace code in script with class

Use a driver class that includes all the setup and time
stepping. Enables easier changes to the code without so much
duplication in the scripts.

 * created `ExtremeFillSystem` class that deals with the setup and
   loop

 * move `write_data` from Variables to `ExtremeFillSystem`

 * renamed the params file to `params.json`

 * renamed `annular.py` to `script.py`

 * most of the code has been moved out of the setup and loop script
   into `ExtemeFillSystem`

 * classes removed from `test.py` and replaced with functions
Move functionality to `ExtremeFillSystem`

Address #14. Tests pass.

Move functionality from `Variables` to `ExtremeFillSystem`. The goal
is to remove the `Variables` class entirely.

 * change `dt` from Variable to float.

 * move methods from `Variables` to `ExtremeFillSystem`

 * move `print_data` from `tools.py` to `ExtremeFillSystem` and use
   Pandas to print the table of values
Residuals as Pandas data frame to stdout

Residuals are printed as a Pandas data frame to stdout. This was
forgotten on the last commit.
Merge branch 'issue14'

Fix #14