become an astropy affiliated package

[lspitler]

Aim is to become an affiliated package. Need to accomplish the following:

• improve user experience to create custom instruments and PSFs e.g. Refactor Imager instantiation #8 Split different Filter types into subclasses #1 Add source spectrum input to remaining signal & noise, SNR, ETC & sensitivity calcs #2
• ensure use of existing astropy packages by fixing a few depreciation warnings and out of date dependencies Getting the LTS test working again #30 a whole lot of warnings when unit tests are run #43
• adopt astropy base package template (almost done) and help determine best workflow for maintenance (e.g. release schedule) update to astropy template #38 pep8 cleanup #40
• provide key useful features: analytical PSF memory error fix Reduce memory usage for simulated images with analytic PSFs #29 ; implement spatially-varying PSF use spatially-varying PSFs #23 PSF wings function to convolve image using Imager PSF #21 physically motivated PSF Implement physically motivated PSF models #4

[lspitler]

@gbpoole this Issue summarises the set of Technical issues that we would like help with via the ADACS MAP.