Add PI construction to Arc3d

[dassaf4]

This is a generalization more suitable to Arc3d. When the instance is a circular fillet, the default inputs yield the arc's PI.

[dassaf4]

@saeeedtorabi Here's an untested implementation. Please generate some tests and verify. One way to verify for circular arc inputs is to call the line string fillet method you recently worked on and try to recover the original line string vertices removed by each fillet arc. Some edge cases to consider:

• f0 === f1
• 0/pi/2pi sweeps (or within Geometry.smallAngleRadians)
• non-circular arc
• non-xy arc