Rename master to main

[fletchapin]

Pull request recommendations:

• Name your branch/PR your-development-type/short-description. Ex: feature/read-tiff-files
• Link to any relevant issue in the PR description. Ex: Resolves [running doctests automagically with Sphinx #12], adds tiff file format support
• Provide context of changes.
• Provide relevant tests for your feature or bug fix.
• Provide or update documentation for any feature added by your pull request.

I went to rename the default branch to main and noticed that main already existed, but diverged from master. I propose that we merge master into main and then set main as the default branch moving forward.

[monroews]

Are there any changes to textbook content with these changes? I'd like to start with the textbook as it currently is published and then verify that these changes are actually beneficial.

The entrance tank is currently published. https://aguaclara-reach.github.io/Textbook/Flow_Control_and_Measurement/ET_Design_Challenge.html

I reread what you are doing and I think it is okay. These are changes are needed so that main is up to date.