ros msg and srv use

New in the package file and folder srv msg, create a new test.msg in both folders, test.srv

Modify apckage.xml

Add the following

<build_depend>：message_generation 
<run_depend>/<exec_depend>：message_runtime

Modify CMakeList.txt

1. When dependent message_generation find_package () was added Construction

find_package(catkin REQUIRED COMPONENTS ...message_generation ...)

1. add_message ()
   to add .msg file

add_message_files(
  DIRECTORY msg
  FILES test.msg
)

1. generate_messages ()
   generates a message header, specify dependencies

generate_messages(DEPENDENCIES std_msgs)

1. catkin_package ()
   relies message_runtime add runtime

catkin_package(
  ...
  CATKIN_DEPENDS ... message_runtime ...
  ...)