ArcBetweenPoints fix/enhancement

[XorUnison]

1. The motivation for making this change (Work on ArcBetweenPoints #40)
   mobject.py has been slightly changed to fix an issue where points that weren't passed into ArcBetweenPoints as np.array caused manim to fail.
   geometry.py had ArcBetweenPoints adjusted to also support radius instead of just angle. Small changes to Arc were necessary to facilitate that .radius returns correct values at all times.

2. How you tested the new behavior:

class Test(Scene):
    def construct(self):
        a = Arc(radius=-1)
        print(a.radius)
        b = Arc(radius=1)
        print(b.radius)
        c = Arc(angle=2)
        print(c.radius)
        d = Arc(angle=1)
        print(d.radius)
        e = ArcBetweenPoints([-1,0,0],[1,0,0],radius=-2)
        print(e.radius)
        f = ArcBetweenPoints([-1,0,0],[1,0,0],radius=1)
        print(f.radius)
        g = ArcBetweenPoints([-1,0,0],[1,0,0],angle=0.1)
        print(g.radius)
        h = ArcBetweenPoints([-1,0,0],[1,0,0],angle=0)
        print(h.radius)

        self.play(Write(VGroup(a,b,c,d,e,f,g,h).arrange(RIGHT)));self.wait(2)

[leotrs]

Thanks! Made only minor comments (that will be caught by black once that is in place), so I'm approving this.

[PgBiel]

shouldnt this be declared in init, or made a private attr (with _failed_to_get_center)?

[PgBiel]

Suggested change

	self.failed_to_get_center=False
	self._failed_to_get_center=False