Doc: Add grouping predict e.g. for lin_reg
Doc: Add grouping predict e.g. for lin_reg

Add example for prediction with grouping support in linear the
regression module.

Closes #209

orhankislal/madlib,iyerr3/incubator-madlib,orhankislal/madlib,madlib/madlib,iyerr3/incubator-madlib,njayaram2/madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,orhankislal/madlib,iyerr3/incubator-madlib,orhankislal/incubator-madlib,orhankislal/incubator-madlib,njayaram2/incubator-madlib,apache/incubator-madlib,njayaram2/incubator-madlib,njayaram2/incubator-madlib,apache/incubator-madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,iyerr3/incubator-madlib,njayaram2/incubator-madlib,njayaram2/madlib,orhankislal/incubator-madlib,hpandeycodeit/incubator-madlib,madlib/madlib,apache/incubator-madlib,orhankislal/madlib,madlib/madlib,njayaram2/madlib,madlib/madlib,orhankislal/incubator-madlib,apache/incubator-madlib,orhankislal/madlib,hpandeycodeit/incubator-madlib,orhankislal/incubator-madlib,hpandeycodeit/incubator-madlib,iyerr3/incubator-madlib,njayaram2/incubator-madlib,madlib/madlib,apache/incubator-madlib