Fix as_rotvec about zero

[bpousett]

Currently, running the following returns a vector of nans:

import spatial_casadi as sc
sc.Rotation.from_quat([0, 0, 0, 1]).as_rotvec()

It should return a zero vector. The issue is in line 423:

scale = theta / casadi.sin(theta * 0.5)

The limit as theta -> 0 of this expression is 2. But this returns nan.

Consider the implementation from scipy spatial, where they use a Taylor expansion to get around this:

        angle = 2 * atan2(_norm3(quat_single), quat_single[3])

        if angle <= 1e-3:  # small angle Taylor series expansion
            angle2 = angle * angle
            scale = 2 + angle2 / 12 + 7 * angle2 * angle2 / 2880
        else:  # large angle
            scale = angle / sin(angle / 2)

Perhaps something similar with ca.if_else would do the trick.