EPL warning needed - now makes it easier to use "=" when you mean ":=".

Since discarding the value of an expression used as a statement was permitted in 10.15.4, one of the situations for a common EPL programming error, using "=" instead of ":=", was permitted. In order to prevent this common error, we have added a warning if this case is detected. This warning will be promoted to an error in a future release.