Feature/msg api improvements

[KerstinKeller]

Description

Rework the general CMessagePublisher / CMessageSubscriber.
Both classes no longer inherit from CPublisher / CSubscriber

All serialization frameworks need to implement a generic Serializer class, which then allows them to easily support all of eCALs framework.

String:

• String classes are no longer templatized.
• Datatype information for String publishers are changed. type=string, encoding=utf8.

Flatbuffers:

• API is now typesafe and more eCAL like. Support different classes depending on the flatbuffer type:
  CObjectPublisher, CObjectSubscriber and CFlatSubscriber

Protobuf:

• Remove DynamicPublisher (Questionable usage).

In a following PR, these Serializer classes will be used to support Measurement read / write access.