nimpa v2.2.0 stable

• add NLM guided filter (#20)
• improc separable convolution fixes & updates (#17, #19)
  • fix memory leak
  • add separable CPU version
  • decrease verbosity
• generate PSF kernels
• fix prc trimming/upsampling bug (#17)
• return file path in imsmooth (#17)
• fix multiarch CUDA support
• improve sitk error handling
• add tests
  • improc CUDA extension (conv & nlm)
  • performance tests (conv & nlm)
• misc build & test framework updates (#17)