Compute standard errors

[boennecd]

Add standard errors for the coefficients as a possible output. The Choleksy decompaction of the inner product of the covariance matrix is computed so this should be straight forward to implement.

[boennecd]

The feature is requested in this Stackoverflow comment: https://stackoverflow.com/questions/23162937/the-rolling-regression-in-r-using-roll-apply/52457815?noredirect=1#comment121401460_52457815