returns log-density of convolution of each component of a normal mixture with N(0,s^2) or s*t(v) at x. Note that convolution of two normals is normal, so it works that way
## S3 method for class 'normalmix'log_comp_dens_conv(m, data)
Arguments
m: mixture distribution with k components
data: a list with components x and s to be interpreted as a normally-distributed observation and its standard error