Update mix.R

Simplified the mix.R by removing _t functions and incorporating their features to the original functions. Functions calling these functions have been modified accordingly as well. 

The list of removed functions are:
postmean_t
comp_postmean_t
postmean2_t
comp_postmean2_t
postsd_t
comp_postsd_t
cdf_post_t
compcdf_post_t
dens_conv_t
compdens_conv_t
comppostprob_t