- Refactored the remove method.

- Replaced with remove_RawContig.

not the old feature table.  So will delete the following features on any contig
which is removed 'cos a clone is deleted:
 - simple_feature, repeat_feature, prediction_transcript, protein_align_feature and
   dna_align_feature.

changed some throws to warnings in MultiTestDB

slice get_repeat_masked_seq made consistent with rawcontig version
updated some slice pods

Now returns null