changes in version 1.7.1
	*modified package functions to handle situations where metabolites are entirely 
	 missing for a particular level of a categorical predictor in the mixture model.

changes in version 1.3.6
	*made small modifications to arguments for function mixnorm
	*added vignette for mixnorm

changes in version 1.3.1
	*added mixnorm function for normalization of non-targeted metabolomics studies