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. #8 #1 #2
• [ ] ensure use of existing astropy packages by fixing a few depreciation warnings and out of date dependencies #30 #43
• [ ] adopt astropy base package template (almost done) and help determine best workflow for maintenance (e.g. release schedule) #38 #40
• [ ] provide key useful features: analytical PSF memory error fix #29 ; implement spatially-varying PSF #23 PSF wings #21 physically motivated PSF #4

[lspitler]

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