* This fixes a bug in concretization. Before the recent change to the
algorithm, the intent was that the @develop version, although
"greater" than numberic versions, is never preferred BY DEFAULT over
numeric versions.
To test this... suppose you have a package with no `preferred=True` in
it, and nothing in `packages.yaml`, but with a `develop` version. For
the sake of this example, I've hacked my `python/package.py` to work
this way.
Without bugfix (WRONG: user should never get develop by default):
```
python@develop%clang@7.3.0-apple~tk~ucs4 arch=darwin-elcapitan-x86_64
...
```
With bugfix (RIGHT: largest numeric version selected):
```
python@3.5.2%clang@7.3.0-apple~tk~ucs4 arch=darwin-elcapitan-x86_64
...
```
* Documented version selection in concretization algo.
* Fix typos
* flake8
98f8f40896