modification to lrt.tcl

The likelihood-ratio test script doesn't work when there are multiple datagroups. The reason is that switching between models by making them active/inactive loses information on the parameters for groups > 1 when the model becomes inactive. We perhaps should modify the model inactive command to track multiple datagroups however as a workaround I modified lrt.tcl to use command files to store the model definitions. Each time the model is switched the appropriate command file is read.