Improve the guard clause in math parsing

This simple test for '<' character, which is always present in
MathML and HTML mathematical formulas, improves performance of
ConceptsCollection::build_from_dataset two-fold.

lib/iev/termbase/term_builder.rb

@@ -327,7 +327,7 @@ def html_entities_to_stem(x)
       end
 
       def mathml_to_asciimath(input)
-        return input if input.nil? || input.empty?
+        return input if input.nil? || input.empty? || !input.include?("<")
 
         unless input.match?(/<math>/)
           return html_to_asciimath(input)