Hub::get_ext_seq method improved

EnsEMBL::Web::ExtIndex remains only an abstract class now, all functionality moved to get_ext_seq method of hub.
Use EnsEMBL::Web::ExtIndex as parent class

get_sequence method added to replace get_seq_by_id and get_seq_by_acc
code used among multiple classes moved to parent class
Rewrite of ENSEMBL_RETRIEVE
Changes according to new Hub::get_ext_seq

Code modified according to the changes made to Hub::get_ext_seq
+ some code tidy
+ use parent