Show how to adjust for batch effects in vignette and documentation

[willtownes]

As a GLM-PCA user with covariates such as batch labels, I can learn how to adjust for these by creating a design matrix and passing it to the GLM-PCA functions. If my design matrix is rank deficient, I receive an error message with instructions on how to fix it so that I can get results that are more numerically stable.

Thanks to Andreas Kapourani for bringing this to our attention.