Add methods for marshalling Localization objects to/from file

[ChrisTimperley]

We should add methods to read and write Localization objects from/to disk. These methods will no doubt be valuable for numerous purposes later in the project, but for now they would be invaluable for easily mocking Localization data. Since, for the time being, we intend to use these methods for the purposes of mocking, we should use a compact and human-readable format to store them; YAML seems like the best fit.

class Localization(object):
   ....
   @staticmethod
   def from_file(fn) -> 'Localization':
     ...

  def to_file(fn) -> None:
    ...