Add double gauss lens assembly

[mattngc]

• Object that groups together lens primitives to form a double gauss lens assembly.
• The spacing of elements is determined by operations on the group rather than operations on individual elements, e.g. zoom, focal length, etc.

This could be implemented as a Node where sub lens elements are simply added to self by the init. Users may wish to manipulate transforms of the sub-elements directly so we won't restrict this. However access is intended to be done via the api.

[mattngc]

This doesn't seem urgent for V0.4. Prefer not to hold up the next release. Moving back to v1.0.