Please add allensdk

[yarikoptic]

Use case: NWB files from .nwb files from Allen's neuropixels dataset (could be obtained from their S3 bucket "natively" or using datalad dataset from e.g. http://datasets.datalad.org/?dir=/allen-brain-observatory/visual-coding-neuropixels). They contain AIBS_ecephys (probably among others) which require first import of the allensdk.brain_observatory.ecephys.nwb for pynwb to be able to load those files.