Begin the transition from INI to YAML

Begin the transition from INI to YAML

* lib/mudpy/data.py(DataFile.load): Temporarily turn this method into a
selector which calls different loader methods depending on the file
extension so that old INI and new YAML files can be intermixed during
the transition.
(DataFile.load_yaml): This new loader method handles YAML files.
(DataFile.load_mpy): This is basically the old DataFile.load method.
(DataFile.save,DataFile.is_writeable): Add some TODO reminders for the
YAML transition.

* lib/mudpy/misc.py(Element.__init__,Element.facets,Element.get)
(Element.getboolean): Add conditional branching around calls deeper into
mudpy.data which need different behavior depending on the underlying
file formats.

* requirements.txt: Start tracking Python module dependencies in this
new file, and add the pyyaml module as the first entry.