repr and str representations for CNMF and Estimates objects

[pgunn]

Hello,
I just landed in the dev branch a change where CNMF and Estimates objects both have informative repr and string representations.

This means that in Jupyter, evaluating either object type will show some information on the state of the object, and if you evaluate either in string context, you'll get a little bit more information yet. Please don't consider either return to be something to parse or a stable API; we may change the formatting of any of these in the future to add or remove information; they're provided as a user convenience.

Current example for a cnm object that has already been through a fit():

>>> cnm
Caiman CNMF Object A.shape=(28900, 149) bg components=2 C.shape=(149, 3000) Use str() for more details
>>> str(cnm)
"Caiman CNMF Object. subfields:['dview', 'dims', 'empty_merged', 'skip_refinement', 'remove_very_bad_comps', 'params', 'estimates', 'mmap_file'] A.shape=(28900, 149) b.shape=(28900, 2) C.shape=(149, 3000)"
>>>