Add scalers

[jm-rivera]

Add scaler functions to /index/data/scalers.py.

From sklearn we could use:

• https://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.StandardScaler.html#sklearn.preprocessing.StandardScaler
• https://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.MinMaxScaler.html#sklearn.preprocessing.MinMaxScaler
• https://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.MaxAbsScaler.html#sklearn.preprocessing.MaxAbsScaler
• https://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.RobustScaler.html#sklearn.preprocessing.RobustScaler

Key questions for an eventual decision:

• Is there a pattern to missing data?
• Are there notable/significant outliers?
• How sparse is the data?
• Do we need to preserve zeros?