Update costmap plugin tutorial (#881)

Signed-off-by: Maurice <mauricepurnawan@gmail.com>

Author: mini-1235

plugin_tutorials/docs/writing_new_costmap2d_plugin.rst

@@ -75,15 +75,17 @@ The basic class provides the set of virtual methods API for working with costmap
 +----------------------+----------------------------------------------------------------------------+-------------------------+
 | reset()              | It may have any code to be executed during costmap reset.                  | Yes                     |
 +----------------------+----------------------------------------------------------------------------+-------------------------+
+| isClearable()        | Method is called to ask the plugin: should it be processed during          | Yes                     |
+|                      | clearing operations or not.                                                |                         |
++----------------------+----------------------------------------------------------------------------+-------------------------+
 
 In our example these methods have the following functionality:
 
 1. ``GradientLayer::onInitialize()`` contains declaration of a ROS parameter with its default value:
 
 .. code-block:: c
 
-  declareParameter("enabled", rclcpp::ParameterValue(true));
-  node_->get_parameter(name_ + "." + "enabled", enabled_);
+  node->declare_or_get_parameter(name_ + "." + "enabled", true);
 
 and sets ``need_recalculation_`` bounds recalculation indicator:
 
@@ -125,6 +127,8 @@ These parameters are defined in plugin's header file.
 
 5. ``GradientLayer::reset()`` method is dummy: it is not used in this example plugin. It remains there since pure virtual function ``reset()`` in parent ``Layer`` class required to be overridden.
 
+6. ``GradientLayer::isClearable()`` returns ``false`` since this plugin is not clearable.
+
 2- Export and make GradientLayer plugin
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