add function to generate tidy dataframe describing cell activity

[dougollerenshaw]

adds a function to data_access utilities to build a tidy dataframe with all cell activity observations.

For example, this:

from visual_behavior.data_access import loading
from visual_behavior.data_access import utilities as data_utilities

oeid = 993891850
session = loading.get_ophys_dataset(oeid)
cell_df = data_utilities.build_tidy_cell_df(session)

gives:

This is a prerequisite for caching cell data in mongo.