[Math] [MATH-1260] SimpleRegression add new method: getPredictionStdErr(double x)

[Patrick Drury]

I have a requirement to be able to get the standard error for a predicted y
value for a given x value. I believe that adding a generalisation of
getInterceptStdErr() to SimpleRegression as detailed in the feature
description of [MATH-1260] would do the trick. Ad hoc experimentation with
exposing xbar via reflection and performing this calculation seemed to
produce good results. I am not a mathematician though, so would be keen to
know if this the correct approach?