[surface code] Organization

[mpharrigan]

This is based off of #1157. See the one new commit for just the changes in this PR until that one gets merged and I rebase.

• Use the names "Beverland et al" and "Gidney Fowler" everywhere
• Split the CCZ2T factory from the functions associated with the model
• rename to "QECScheme"
• Remove global variables, which weren't pep8 names and the names would have been too many capital letters. Port them to consistently-named factory methods on their respective classes.
• Relative imports in __init__.py

[mpharrigan]

This is rebased and ready for review