GH-#518 Add NWB epoch support for epochs saved from MIES in NWB

[MichaelHuth]

When loading NWB files exported with MIES then the epoch information is parsed in ipfx as well.

The API stays compatible, epochs can be retrieved as before when the sweeps object is constructed. Epoch names from nwb are prefixed with "nwb:" to set them apart from the already existing epochs.

So for epoch "E0" in MIES the epoch name in ipfx is "nwb:E0".

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Close #518

Type of Fix:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing
  functionality to not work as expected)
• Documentation Change

Checklist

• My code follows
  Allen Institute Contribution Guidelines
• My code is unit tested and does not decrease test coverage
• I have performed a self review of my own code
• My code is well-documented, and the docstrings conform to
  Numpy Standards
• I have updated the documentation of the repository where
  appropriate
• The header on my commit includes the issue number
• My code passes all tests
• I have updated the CHANGELOG.md with the description of changes understandable by end users