HoloViews: Short term refactoring improvements

• A new utility:

def get_common(obj, attribute, default, 
               ignore_auxillary=True, disallowed=[]):

Takes an object, an attribute string, a default (to return if not all common), ignore_auxillary to include or exclude auxillary elements, and a disallowed list such that the default is returned if the return value is disallowed.

• This should get rid of a lot of code used to check if label/group/bounds are common or not across collection types.