Write data generator for denoising

[danielsf]

This is an attempt to split up #374 into manageable chunks.

For the purposes of writing a denoising module that Pika owns, we want a class that can read in a 2-Photon movie or set of 2-Photon movies and iterate over the frames, grabbing the chunks of frames needed for deepinterpolation.

Requirements

• Class must accept a parameter specifying the number of frames to grab for each deepinterpolated window (the default is 30 frames before and after the frame of interest)
• Class must supply frames/windows to tensorflow as requested by the the tensorflow API during training/inference (by implementing __len__ and __getitem__)
• Class should subclass keras.utils.Sequence

This will be a requirement of #469