merge master and develop into main

[orbeckst]

The master/develop branch model does not really reflect how we develop so we want to just have a single main branch that "always works". We then cut releases directly from main.

• merge master and develop
• rename to main
• change default branch on GitHub to main
• change branch on RTD to main

[orbeckst]

Pushed out so that open PRs and forks don't get upset; I'd rather move along than add more friction.