allow regress out covariates for scVI intergration

[fentouxungui]

add 3 new parameters.

1. meta.data.df: the meta.data from seurat object, default NULL, only works if any categorical.vars.to.regress or continuous.vars.to.regress is not NULL.
2. categorical.vars.to.regress: the covariates from meta.data of seurat objec, columns should be factor type. default NULL.
3. continuous.vars.to.regress: the covariates from meta.data of seurat objec, columns should be numeric type. default NULL.
   if you do not want to regress out any covariates, just leave all these 3 parameters default.