add Variably Methylated Regions (VMR) to DMR pipeline

[marcmaxson]

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4210618/

DiffVar: a new method for detecting differential variability with application to methylation in cancer and aging
Methylation of DNA is known to be essential to development and dramatically altered in cancers. The Illumina HumanMethylation450 BeadChip has been used extensively as a cost-effective way to profile nearly half a million CpG sites across the human genome.