Edit User metric warning if use sample() function

.. its much faster to vectorize and use the samples() function in init
   or value function when working with user metrics.

   for example this code:

   sample {
      work <- work + (POWER * recIntSecs);
   }
   value { work; }
   count { Duration; }

   should be refactored to:

   value {
      work <- sum(samples(power) * recIntSecs);
   }
   count { Duration; }

src/Charts/LTMTool.cpp

@@ -2725,6 +2725,8 @@ DataFilterEdit::checkErrors()
     QStringList errors = checker.check(toPlainText());
     checker.colorSyntax(document(), textCursor().position()); // syntax + error highlighting
 
+    if (checker.rt.functions.contains("sample")) errors << tr("Warning: sample() is slow -- update code to use samples()");
+
     // even if no errors need to tell folks
     emit syntaxErrors(errors);
 }