'Cache for this frame, a dict.  It stores anything that should be
computed from the coordinate data (*not* from the frame attributes).
This can be used in functions to store anything that might be
expensive to compute but might be re-used by some other function.
E.g.::
if \'user_data\' in myframe.cache:
data = myframe.cache[\'user_data\']
else:
myframe.cache[\'user_data\'] = data = expensive_func(myframe.lat)
If in-place modifications are made to the frame data, the cache should
be cleared::
myframe.cache.clear()'

@lazyproperty
def cache(self):

    return defaultdict(dict)
