Change the name Database3 to Database

[john-science]

This is a big, complex change that will have to be done in phases.

It is an unfortunate fluke of history that ARMI's database class is called Database3. This was meant to be a temporary name used during a transition.

But, obviously, this is a big, API-breaking change that will cause a ton of (trivial) code changes in ARMI and downstream. So, I would suggest a long, gradual transition:

• Move database3.py to database.py, but create the stub file below in a new database3.py.
• Slowly transition all code in ARMI and downstream to use Database (without the "3").
• Eventually, add a DeprecationWarning in Database3.py
• Finally, eventually, remove Database3 from ARMI.

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Suggested stub file to ease our transition:

# database3.py
from armi.bookkeeping.db.database import *

class Database3(Database):
    pass