v11.7.1: rerwite fit.lognormal function.
v10.5.6: add a test to check whether a gene is marginally expressed or 
  not expressed. A fnmm will be fitted only if evidence shows that the 
  gene is expressed at a measurable level.  Otherwise, no further tests
  need to be performed for DEG detection.
v10.5.1: rewrite perm.test.NGS function for user-defined functions to 
  compute statistics.
