N epoch test

[martinkilbinger]

Summary

In the PSFEx and MCCD interpolation modules, a check is added for the input key 'N_EPOCH'. A KeyError is raised if not present. Closes #626 .

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[martinkilbinger]

@lbaumo when you get a chance you can check this PR and accept (if everything is ok). The CI tests are failing due to the pysap issue, but we can ignore this here since only the documentation changed.