Investigate and possibly resolve code duplication in `Circle`

Currently, Circle extends from CircleMarker, and overrides certain methods to use a custom _mRadius property for the radius. However, it seems that the logic is largely comparable to the super-class, which uses its own _radius property. In fact, both these properties are used within the Circle class, somewhat interchangeably.

As part of this issue, we need to determine if the logic in Circle is actually the same as CircleMarker and we can unify the _mRadius and _radius properties, and if so, unify them into a single property, eliminating redundant methods in the process (e.g. getRadius(), setRadius(), setStyle(), etc.).