ENH: Add method to tell user what needs to be passed to find_granules and friends
ENH: Add method to tell user what needs to be passed to find_granules and friends

Added a method pyatmlab.dataset.Dataset.get_path_format_variables.
This method informs the user what additional parameters need to be passed
to iterate_subdirs, find_granules, etc., in cases where such additional
parameters are needed.  For example, HIRS exists on multiple satellites,
so `satname` needs to be passed.

olemke/pyatmlab,gerritholl/pyatmlab