Add data count to tfrecords

[hvgazula]

We decided to add an extra feature to each record/example labeled "data_count". While we do this, we also need to add logic to adjust the number of volumes in each epoch (in case drop_reminder is set to True during batching). This is also important because the bayesian meshnet requires the number of examples upfront. See https://github.com/neuronets/nobrainer/blob/976691d685824fd4bba836498abea4184cffd798/nobrainer/models/bayesian_meshnet.py#L20