Add webbpsf and poppy to environment

[ojustino]

I am proposing that webbpsf and poppy be added to the base stenv environment. They can be found at https://github.com/spacetelescope/webbpsf and https://github.com/spacetelescope/poppy.

• This package is useful or relevant to a significant amount of stenv users.
• This package does not require specific versions of other packages in stenv (inclusion would not introduce
  significant backward constraints to the requirements).
• This package's tests do not take an overly long time to complete, and test data files are not significantly large.
  • The full data files for the latest version of webbpsf are ~70 MB. A minimal variant that's been downsized to ~50 MB is also available.

Additional packages / requirements:

webbpsf requires:

• numpy
• scipy
• matplotlib
• astropy
• photutils
• poppy
• pysiaf
• synphot (already in stenv)

poppy requires:

• numpy
• scipy
• matplotlib
• astropy

To add a new package, the requirement should be added to the *.yml files defining the environments. For instance, for the base environment, stenv_latest.yml, stenv_dev.yml, and stenv_stable.yml. Then, the new package's tests should pass against the built environment.