bug fix (from Marc Lamarine): returned Bio::Seq display_id was not set consistantly by Exon::seq method

Added my $sub_slices = split_Slices($slice,$maxlenght,$overlap) type methid to enable the removal of the maxlength and overlap from the sliceAdaptor->fetch_all method

added some more sanity checking to SliceAdaptor::fetch_by_region - cannot create slice on non-existant or toplevel CoordSystem

Examples for creating and reading density features.  Three types are given. (%GC, %repeat and gene count)

Toplevel setting should work now.  Added rank to all coordinate systems. Fixed Danio gene transfer code as well.

modifications so that toplevel is now a pseudo coordinate system representing the highest real coordinate system in a given region

changes to make elegans xrefs work

fixed bug in which undef was being returned from rearrange when zero should have been returned instead

Superseeded by density_feature and density_type