make the flexible way to accept database information as well as analysis and contig, during the conversion of seqfeature a sort from bioperl to ensembl.

to the existing "isa", and added a friendly throw to
AUTOLOAD if the method does not exist on the object.

external_db db_name enum

more doc for converter factory to enumerate all instances so far.

changed some if defined to ifs given 0's and null string shouldn't be evaluated as true in those
contexts