Renaming Database3 to Database, but preserving the API

[john-science]

What is the change?

Here I rename Database3 to Database everywhere in ARMI.

However, I preserve the old Database3 end point, as a temporary measure, so this PR is full backward compatible.

Why is the change being made?

This PR makes progress on: #1673

But this PR will not close that issue, because I preserve the old database3.Database3 API end point. This is a temporary measure to ensure backwards compatibility in projects downstream.

The goal is to give downstream projects time to make the change.

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Checklist

• This PR has only one purpose or idea.
• Tests have been added/updated to verify any new/changed code.

• The code style follows good practices.
• The commit message(s) follow good practices.

• The release notes have been updated if necessary.
• The documentation is still up-to-date in the doc folder.
• The dependencies are still up-to-date in pyproject.toml.

[john-science]

@opotowsky This is another change that is fully backwards compatible and won't require any downstream changes yet. It provides time for downstream projects to conform.

But last time I tried this it required somewhat more finesse than I expected, so perhaps we should talk offline to make sure I do all my i's and cross all my t's?