TRACKING: Installation methods for users & developers

[neiljp]

This issue is a spin-off of #269.

Ideas raised so far:

• Snap Improve Installation notes for users & developers #269 (comment)
• Tox [for development; allows testing multiple python versions & package installs]
• Pipenv (keep including it? just for developers? prioritize? just use pip -e .[dev]?)

[neiljp]

homebrew: This was discussed in #320, with the conclusion that we would rely on pip for now.

[neiljp]

We are currently focused on supporting:

• user installation from pypi (pip)
• user installation from latest git (pip)
• user installation from pypi (docker, requiring building docker image)
• user installation from latest git (docker, requiring building docker image docker: Add dockerfile for building images from latest code (upstream). #571)
• developer installation (pipenv)
• developer installation (pip)
• developer installation (pip+make, as of Support development using makefile #777)

Unless there have been marked changes to pipenv recently, my preference would be to remove it to simplify the user experience and minimize the maintenance overhead (we don't test installing with pipenv in CI any more), and refocus on pip+make, with a detailed pip approach as backup.

We could likely improve our dependencies versioning, but that's another issue, which could highlight the importance of pipenv or pip-tools, or even a move to eg. poetry.