Support constructing `CustomJoint` and `SpatialTransform` objects in scripting

[nickbianco]

The following snippet,

spatial_transform = osim.SpatialTransform()
joint = osim.CustomJoint('ground_pelvis', model.getGround(), pelvis, spatial_transform)
model.addJoint(joint)

leads to a segmentation fault in Python, likely because the SpatialTransform is provided as a modifiable reference to the CustomJoint constructor.

I've also had issues creating custom SpatialTransforms from scratch directly in Python. This will be valuable for model building via scripting.