Refactor: change magic constant 0.6 to property cfl.

This change is needed to allow client to change CFL number used
to compute stable time step.
Fix a bug in computation of the spatial step.

As we compute the spatial step in terms of the internal points
of the domain, then we should add 1. Look at the picture:
                    |---*---*---*---|
From the above line it's clear then if we have 3 internal points,
then we have 4 intervals separated by these points.
Fix a bug with periodic boundary conditions.

The bug is an "off by 1" bug. Instead of equaling leftmost and rightmost
points of the domain, code was equaling leftmost and left to the
rightmost points.
Refactor: replace expressions with variables.

Indices of the leftmost and rightmost points were defined as object
variables. So it's natural to replace the same expression everywhere in
the code with these variables.
Refactor: rename npoints to ninternal_points in constructor.

ninternal_points is much better conducts the meaning of the variable
than npoints.
Reformat code for readability.
Add folder data to the repository.
Revert "Reformat code for readability."

This reverts commit 4b4f015ce77ca113a971b7fef53bce04f11fc1d9.
Rename dummy file.
Add data folder to .gitignore.
Add convergence test for decoupled system.