rationalize SpaceTelescopeInstrument.calcPSF with poppy.Instrument.calc_psf

[mperrin]

This got overlooked in the API updating. Whoops...

See https://github.com/mperrin/poppy/issues/180#issuecomment-249695186 :

just realized to my horror that when we updated the API for calcPSF->calc_psf we never updated SpaceTelescopeInstrument. So I've been doing all my testing lately calling calc_psf which was just calling the parent Instrument.calc_psf. It turns out that has been working fine - so I'm now confused and can't remember why we overrode that function in SpaceTelescopeInstrument.

[mperrin]

After fixing a trivial typo in a warning string in poppy.Instrument, it seems that most webbpsf tests are passing with calls to Instrument.calc_psf instead of SpaceTelescopeInstrument.calcPSF (hereafter STI.calcPSF for brevity).

What's different between those two functions?

• STI.calcPSF has support for a couple deprecated input parameters which we can just do away with now.
• STI.calcPSF allows specification of rectangular rather than square fields of view. We should move this bit of useful code to Instrument.calc_psf.
• if oversampling is not explicitly set, STI.calcPSF reads a default from the package configuration settings, while Instrument.calc_psf has it hard-coded to 4.
• if display is True, STI.calcPSF has slightly different plot annotation text than Instrument.calc_psf does.

None of those seem sufficient to justify separate functions at this point. I think this is pretty much just vestigial code and we can & should simplify it down to just one function.

[mperrin]

Fixed (in the si_wfe branch for now, will merge later to master.