Sugestion for a new grammar for the message specifications

[ipa-nhg]

These changes are from @ipa-rwu
This PR include the remaining commits from #194
‼️ Code has to be re-generated before merge ‼️

This suggestion requires a major discussion. Initially, the idea was to keep the .msg format for the definition of the specifications, for example:

std_msgs:
 specs:
  msg: ColorRGBA
    message:
       float32 r
       float32 g
       float32 b
       float32 a
std_srvs:
 specs:
  srv: Trigger
    response:
       bool sucess
       string message

This approach will significantly lower the entry barrier for the ROS community. As the original code is:

std_msgs/ColorRGBA.msg

float32 r
float32 g
float32 b
float32 a

std_srvs/Trigger.srv

---
bool success   # indicate successful run of triggered service
string message # informational, e.g. for error messages

With this PR you are proposing:

std_msgs:
  specs:
    - msg: ColorRGBA
      message:
        r: float32
        g: float32
        b: float32
        a: float32
std_srvs:
  specs:
    - srv: Trigger
      request:
      response:
        success: bool
        "message": string