Add remove method for deleting a variable

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

Author: JaredDavis22

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

@@ -95,6 +95,11 @@ public void setAll(final Map<? extends String, ? extends Object> map) {
 		vars.putAll(map);
 	}
 
+	@Override
+	public Object remove(final String name) {
+		return vars.remove(name);
+	}
+
 	@Override
 	public void clear() {
 		vars.clear();

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

@@ -191,6 +191,15 @@ default void set(final Variable v, final Object value) {
 	 */
 	void setAll(Map<? extends String, ? extends Object> map);
 
+	/**
+	 * Removes the named variable.
+	 *
+	 * @param name The name of the variable whose value you want to remove.
+	 * @return The previous variables value associated with name,
+	 *         or null if the name did not exist.
+	 */
+	Object remove(String name);
+
 	/**
 	 * Clears all the variables.
 	 *