Change snp caller interface to use rods exclusively

As mentioned in issue #9 the SimpleSNP caller and MPileupSNP caller differ in their callSNP functions so the parent-level call  function on RDD[ADAMRead] only uses the pileup one in SimpleSNP and not the rod one in MPileup.  This changes both to use rods.

Hopefully this will be helped further by changes to the pileup interface.