Improve customizability and usability of data pipelines

[adarob]

I'm opening this to address points raised in #142 and #145.

2 clear action items raised by those issues are:

1. Make it easier to add new features. Ideally these would be added in one place (e.g., the DataProvider) instead of 2 (i.e., the DataProvider and prepare_tfrecord_lib).
2. Create a tutorial demonstrating how to add features and use custom tfrecord files.

[barisbozkurt]

Thanks for the efforts. If view/testing of an outsider would be helpful, I volunteer.