Implement seasonality into RAMP directly

[Bachibouzouk]

There have been successful attempts at modelling seasonality with RAMP core methods, however this feature is highly interested for many usecases and it would make sense to introduce it into RAMP.

What specifically need to be discussed here is

1. What are the typical inputs needed to provide seasonality ?
2. How are those inputs conveyed to RAMP using the existing excel file input ?
3. What is to be changed inside RAMP core ?

[Bachibouzouk]

@JW-Kraft, @mohammadamint @FLomb