Add metric meter for accessibility

I have added a metric meter for the accessibility metric and generalized
the code to dynamically create the metric graphic.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add accessibility metric skeleton

Just added the class for the accessibility metric computation in order
to write a failing test in the next step.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>