test all parameter combinations

[aaronspring]

currently only defaults are tested

cat.atmosphere.GISTEMP(t_res='ltm').describe()
'user_parameters': [{'name': 's_res',
   'description': 'spatial resolution',
   'type': 'str',
   'allowed': ['1200km', '250km'],
   'default': '1200km'},
  {'name': 'realm',
   'description': 'data region including or excluding ocean',
   'type': 'str',
   'allowed': ['landonly', 'combined'],
   'default': 'combined'},```