Establish data samples: producers + tester pairs

[yarikoptic]

Types of files:

• basic nwb file with metadata (like we have those simple ones in dandi-cli tests fixtures)
• domain specific nwb files probably following the tutorials, such as https://pynwb.readthedocs.io/en/latest/tutorials/domain/icephys.html#sphx-glr-tutorials-domain-icephys-py
• files with NWB extensions , which are listed on https://nwb-extensions.github.io/

data sample "interface"

• producer save(filename: str): would create and save (into a provided filename) the file
• tester test_*(object: ?): given the loaded file, verify that it loaded according to what was expected -- use assert

nwb_healthstatus/
  samples/
     <data_producer: {core,ephy_testing_data,extensions}>
     extensions/<extension_name>/
        sample1.py

Existing data producers

• https://github.com/SpikeInterface/spikeextractors/blob/master/tests/test_gin_repo.py - which operates on (non-nwb original files) https://gin.g-node.org/NeuralEnsemble/ephy_testing_data