Output fixes, str vs repr, exceptions and type hints

Sorry, a big diff for many changes, but they were all linked 😅

• Do not setattr(self, parameter) on App obj anymore
• Refac save_objects and data - fixes #75 (closed) #72 (closed)
• Using save_objects, App.parameters (and summarize()) is now correctly in sync with otb.Application parameters
• Move summarize() back to App
• Fix bugs with Output, __getitem__ in OTBObject and Output[slice], summarize() call parent pyotb app.
• Add App.outputs to access Output objects via App[param_key] (see __getitem__)
• Rename key_input and key_input_image to input_key and input_image_key
• Raise TypeError when get_first_key failed, fixes #71 (closed)
• Rename write() arg filename_extension to ext_fname, fixes #77 (closed) and error when "?" is not passed: add missing "?&"
• Add App.output_key
• Remove App.image_dic : fixes #79 (closed)
• Type hints
• Update tests, move data dicts to tests_data
• Use __repr__ in OTBObject so user is free to implement __str__, add missing hash and id dunder methods
• Merge summarize tests in test_core, and move summaries data to json file

The diff is positive because of test json data for summarize() - output is now far bigger, since App.parameters is really in sync with otb.Application instance.

TODOs from Remi's comments

• TBD: Path and local files stuff removal (my thought) or at least disabled when using /vsi prefix
• Add tests for the improved getitem behavior
• Keep using otbApplication's enums for pixel types, parameter types, ...
• Fix summarize (back to same state than in current develop branch)