ThinLens update to use explicit radius argument

[josePhoenix]

Note: this remains a work-in-progress. WebbPSF master does not work for the commit I based this branch at, so there's something else going on.

Per mperrin/poppy#15, poppy.ThinLens now requires a radius argument to be given explicitly.

Some of the corresponding changes are:

In WebbPSF:

• JWInstrument gains a pupil_radius attribute from the poppy.FITSOpticalElement generated in _getOpticalSystem
• NIRCam thin lenses gain a radius keyword argument where they are instantiated
• the ThinLens created when defocus_waves is supplied as an option gains a radius keyword argument

In POPPY:

• addPupil, addRotation, addImage, addDetector all return the element they add (allowing API users to retain a reference to any FITSOpticalElements or others created transparently from their arguments)

[mperrin]

Whoops, could you pull request also the related changes in poppy too?

[josePhoenix]

Alas, the required branch is currently only on my work computer. Will do tomorrow!