SPTP11 — PAstep needs tags for message size

There are no tags for message size, which is really important when the
messages are
transmitted over a communication network. For a synchronous message, there
are in
fact two message sizes: one for the request and the other for the reply.

• As the UML messages are stereotyped as PAStep, an easy fix would be
  to add
  two tags to a step, PAinSize and PAoutSize of type Integer, to describe the
  size of
  the data needed as input for the step, and the size of the results produced
  by the
  step. Most steps would not need these tags.