add "datasets" module

[neuromusic]

this module should implement a similar api as sklearn.datasets and nilearn.datasets

• use sklearn.datasets.base (including sklearn datasets cache folder) to store downloaded data
• use logic as in crcnsget to download data (optionally using environment variables for passwords)
• functions should return data ready to analyze with neuroglia (e.g. event dataframes, spike dataframes, or trace dataframes/xarrays)

first candidate datasets, depending on needs for examples:

cai-1 for calcium inference

• https://portal.nersc.gov/project/crcns/download/cai-1
• used in http://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1005423

allen institute brain observatory experiment for decoding example