Add clear function to complete map operations

Signed-off-by: Curtis Rueden <ctrueden@wisc.edu>

Author: JaredDavis22

src/main/java/org/scijava/parsington/eval/AbstractEvaluator.java

@@ -95,4 +95,9 @@ public void setAll(final Map<? extends String, ? extends Object> map) {
 		vars.putAll(map);
 	}
 
+	@Override
+	public void clear() {
+		vars.clear();
+	}
+
 }

src/main/java/org/scijava/parsington/eval/Evaluator.java

@@ -191,4 +191,10 @@ default void set(final Variable v, final Object value) {
 	 */
 	void setAll(Map<? extends String, ? extends Object> map);
 
+	/**
+	 * Clears all the variables.
+	 *
+	 */
+	void clear();
+
 }