Allow PoseGenerators to generate a greater variety of poses

[schornakj]

• Add new parameters to PoseGenerator implementations for rotation around camera Z-axis and transform offset relative to target origin.
• Allow multiple PoseGenerators to be used to create observations when setting up a given test case.
• Pass around SharedPtrs to PoseGenerators (needed to handle collections of PoseGenerators, since the base class is abstract).

[schornakj]

The extremely tentative initial result for the hemisphere test case is that adding a second set of observations at the same poses but rotated by 0.5*pi around the camera-local Z+ axis reduces the correlation between the translation parameters of the camera and wrist transforms, but increases the correlation of some of the rotation parameters.

Original case (no rotation):

Covariance (camera to wrist):

	x	y	z	rx	ry	rz
x	0.0669069	0.595029	0.578816	0.277428	0.346459	-0.293642
y	0.595029	0.0536759	0.545875	-0.0104255	-0.43186	0.366025
z	0.578816	0.545875	0.0655187	0.815311	-0.227154	0.192525
rx	0.277428	0.0104255	0.815311	0.0924252	-1.08837e-12	-2.14972e-12
ry	0.346459	-0.43186	-0.227154	-1.08837e-12	0.0771227	-0.709471
rz	-0.293642	0.366025	0.192525	-2.14972e-12	-0.709471	0.0282724

New case (more poses rotated by 0.5*pi):

Covariance (camera to wrist):

	x	y	z	rx	ry	rz
x	0.021531	0.0579856	0.145417	-0.0841606	0.391177	-0.0988148
y	0.0579856	0.027871	0.0507486	-0.701381	-0.791442	0.621118
z	0.145417	0.0507486	0.0211778	0.595235	-0.277889	-0.190204
rx	-0.0841606	-0.701381	0.595235	0.045453	0.356191	-0.561755
ry	0.391177	-0.791442	-0.277889	0.356191	0.0457319	-0.488505
rz	-0.0988148	0.621118	-0.190204	-0.561755	-0.488505	0.016959

[schornakj]

I restructured the PoseGenerator struct to allow fully-randomized rotation around camera Z+.

I'll address the implications for covariance in an upcoming PR.

@marip8 Ready for review when you get a chance.