First cut of more object-oriented approach.  Classes are in
snp_classes.py
Continue making code more object-oriented.  Recognized that in addition
to chromosome numbers, we can have values such as X, Y and MT.  So the
selection parameters no longer use a chromosome number range, but a list
of chromosomes to select from.  If unspecified all chromosomes are
included.  Also, renamed methods according to python style guide (using
underscores to separate segments.)