Support for multiplicative effects for arbitrary covariates in the spatial data

[vitkl]

Addresses #95

This feature enables accounting for various covariates such as donor, age, potentially batch.
This parameter is added as a separate multiplicative parameter y_{t,g} with regularising prior centered at 1. E.i. exactly as technical/extra categorical covariates in the regression model.

\mu_{s,g} = (m_{g} \left (\sum_{f} {w_{s,f} \: g_{f,g}} \right) + s_{e,g}) y_{s} y_{t,g}