Prediction interval

adding `get_prediction` to linear_model RegressionResults

everything is written in standalone function for reuse in other models, but some things are linear model specific. (It's not a generic function.)

main issue for regression.linear model #719 
also several discussions on mailing list

SUMMary issue for extension to other models #2150 
some implementation discussion for wrapping #2145 


**Status**

so far mainly the basic outline, I'm trying to get that settled first
I didn't check much of the details yet. 
unit test against old sandbox function.