I290 remove aig support

[eyvorchuk]

Changes

• Removes pydap.responses.aaigrid requirement in requirements.txt and setup.py
• Removes pydap.responses.aaigrid.rst and associated reference in index.rst
• Removes aig references in pdp_controls.js and pdp_download.js
• Removes aig tests in test_functional.py
• Specifies reason for removing aig support in raster.rst and faq.rst
• Changes database strings in env files in docker deployment directories from db3 to db. This is technically outside the scope of the PR, but I figured I'd do it sooner than later.

Resolves #290

Checklist

• [x] Full integration tests are passing locally
• [x] Docs have been updated to reflect the changes

[rod-glover]

Oops, wait, I found some things worth asking about.

[rod-glover]

Searching revealed one more mention of aig.

• https://github.com/pacificclimate/pdp/blob/master/pdp/static/js/prism_demo_app.js#L25-L33