Added resume function that autodetects class and file type.

Summary:

1.  `resume()` is no longer a classmethod, but a straight up function.
2.  `resume` is imported at the top level of __init__.py, so we can do `from repex import resume` or `import repex; repex.resume()`
3.  `resume()` looks up the subclass name from the netCDF and loads the appropriate constructor appropriately
4.  General cleanup
5.  `create_repex()` is now `create()`