518 native + refactored docker installer + docs

[ra-tolson]

Fixes #518. Presently this branch rests atop the fix for #515; the former should merge first.

This has been rebased after #516 merged; stand by.

[ra-tolson]

PS - I need to add this is a 'first iteration': It solves the problem just enough to have a native installer for python 3 + gippy 1.0. It doesn't:

It does do these things now:

• install sixs
• refactor the dockerfiles to call into the installer

It doesn't:

• make use of any of the advances in the dhscratch2 branch

[ra-tolson]

I have some more commits incoming; I think reaching CI pipeline passage using the new installer would be a nice checkpoint.

[ra-tolson]

See CI pipeline 90721435 for passing the test suite. Current state:

• Breaks installation scripts into three parts (system dependencies, python dependencies, and gips installation proper); this way hopefully we (and users) can call the bits separately if they wish (next step is virtualenvs for instance).
• Thus you can take or leave the bits of system installation you need for your use case.
• since pip3 could be installed already, it checks first before assuming ubuntu package is wanted.
• intentionally broke things into short nearly logic-free scripts instead of trying to write installer logic in bash because bash is very hard to get right.

[ra-tolson]

That commit describes how to use the install scripts & how to do up a virtualenv for users that want that. I also ran the full test suite locally (everything but fetch & modisndti) and it all worked. Also I ran a modis fetch to confirm earthdata etc continues to function.