changes in version 1.9.1
	*modified package so that normalization function does not output data 
	when any model parameters are not possible to estimate. 
	
	*added function metabPlot to plot pre vs. post normalization 
	metabolite abundances. 
	
	*updated mixnorm vignette

changes in version 1.7.3
	*modified package functions to handle situations where metabolites are entirely 
	 missing for all levels of a categorical predictor in the mixture model.
	 
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