New module, inherits from both formats
Change BAM to use new glyphset if it's an internal source
More work on combined BAM/BigWig tracks
Get new glyphset to call correct methods
Minor efficiency improvements - use cached filehandle if available, and don't munge chromosome names (i.e. add/remove 'chr') if the species doesn't have chromosomes!

The latter needs a flag passed in by code calling this method, e.g. in GlyphSet::bigwig, which has access to the EnsEMBL::Web::Hub object
Extended adaptor method to work with database-configured BigWig files, e.g. overviews of RNASeq data. (Also removed some debugging warns.)
Don't forget to pass the bins parameter to the method, otherwise there's a divide by zero error and Apache coredumps!