TYP: NDFrame.(loc|iloc|at|iat)
Currently, the NDFrame indexers (.loc/.iloc/.at/.iat) are too complex set up to let mypy understand them. This PR makes their implementations understandable for mypy (and humans also maybe, the old imp. was a bit indirect so took som effort to understand).

One issue is that I  - for the life of me - can't programatically set doc strings of properties and make mypy understand the properties' types at the same time. Python/mypy seem to demand that doc strings be set directly on the property if the type is to be understandable by mypy. E.g. Appender on a property is not supported by mypy. So I've set the doc string on the NDFrame properties, and reference them from the indexer classes instances. This makes the PR seem large, but it's mostly just moving the doc strings.
