Added group_id, level_id methods as well as hslice. Modified in consequence the invert method to set hslice
Added group_id, level_id methods as well as hslice. Modified in consequence the invert method to set hslice

mjg17/ensembl,willmclaren/ensembl,Ensembl/ensembl,at7/ensembl,Ensembl/ensembl,mjg17/ensembl,danstaines/ensembl,at7/ensembl,james-monkeyshines/ensembl,muffato/ensembl,at7/ensembl,at7/ensembl,danstaines/ensembl,willmclaren/ensembl,mjg17/ensembl,james-monkeyshines/ensembl,muffato/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,muffato/ensembl,muffato/ensembl,mjg17/ensembl,Ensembl/ensembl,Ensembl/ensembl,james-monkeyshines/ensembl