Added missing import

Created by: muffato

Description

Without the import, throw doesn't work

Use case

Call alignment_strings on a DnaDnaAlignFeature that is not of ensembl type and you'll get

Undefined subroutine &Bio::EnsEMBL::DnaDnaAlignFeature::throw called at /home/matthieu/workspace/src/rel91/ensembl/modules/Bio/EnsEMBL/DnaDnaAlignFeature.pm line 262.

Benefits

with the import, you'll get the much nicer

-------------------- EXCEPTION --------------------
MSG: alignment_strings method not implemented for CACTUS_HAL_PW
STACK Bio::EnsEMBL::DnaDnaAlignFeature::alignment_strings /home/matthieu/workspace/src/rel91/ensembl/modules/Bio/EnsEMBL/DnaDnaAlignFeature.pm:263
STACK toplevel HAL/HALXS/test_anne.pl:192
Date (localtime)    = Thu Nov 30 16:20:36 2017
Ensembl API version = 91
---------------------------------------------------

Possible Drawbacks

Every change breaks someone's workflow

Testing

Have you added/modified unit tests to test the changes?

No

Have you run the entire test suite and no regression was detected?

No