Add TFRecords creation to OTBTF

We would need to define a new class TFRecord that handles all the dataset <-> tfrecord conversions. Inside otbtf.py ? Or we split otbtf.py into a python module:

otbtf
|───dataset.py
|───tfrecord.py
└───utils.py

Maybe add a method to_tfrecords(out_dir) inside Dataset class ?