replace delete_if with select

to avoid modifying the original array, which prevents issues with frozen arrays,
fixes #161 "FrozenError - can't modify frozen Array"

helpers/slices_helper.rb

@@ -10,10 +10,8 @@ def filter_for_slice(obj)
 
         slice = current_slice()
 
-        if obj.is_a?(Enumerable)
-          if obj.first.is_a?(LinkedData::Models::Ontology)
-            obj.delete_if {|o| !slice.ontology_id_set.include?(o.id.to_s)}
-          end
+        if obj.is_a?(Enumerable) && obj.first.is_a?(LinkedData::Models::Ontology)
+          obj = obj.select { |o| slice.ontology_id_set.include?(o.id.to_s) }
         end
         obj
       end
@@ -35,4 +33,4 @@ def current_slice_acronyms
   end
 end
 
-helpers Sinatra::Helpers::SlicesHelper
+helpers Sinatra::Helpers::SlicesHelper