Changes to _slice_fetch: simpify SQL if slice is an SNP - only 1bp long.

Functionality suggested by Ensembl Genomes, used when building a variation DB.
New flag: start_equals_end which controls the simplified sql for 1bp slice.

modules/Bio/EnsEMBL/DBSQL/BaseFeatureAdaptor.pm

@@ -87,7 +87,7 @@ sub new {
                to true sub _slice_fetch will use a simplified sql to retrieve 1bp slices.
   Returntype : boolean
   Exceptions : none
-  Caller     : Pipeline
+  Caller     : EnsemblGenomes variation DB build
   Status     : Stable
   
 =cut