Support several (optional) normalizations

[nshaud]

Currently the dataset is normalized into (0,1). This is mostly fine but we should be able to use other normalizations or even no normalization at all.

TODO:

• [ ] Add a normalization option
• [ ] Support multiple normalizations (to define)

[nshaud]

@jfrob27 what sort of data normalization do you use on astro data?

[mengxue-rs]

Please see https://github.com/nshaud/DeepHyperX/pull/38#issue-532363161