Issue40 core api

[erikzenker]

This PR:

• Divides the communication policy API into core and extended API
  • Core API are methods that are required by hard (point to point communication)
  • Extended API are methods which can be emulated by core API methods, but can also be provided in optimized variants e.g. MPI collectives
  • A communication policy which only implements the core API can inherit a emulated extended api from graybat::communicationPolicy::Base<CommunicationPolicy>
  • Implemented by the Curiously recurring template pattern
• Introduces Type traits for communication policy (necessary since core/extended api) which also define the communication policy interface more clear
• Introduces configuration objects which are used in instantiate policies
• Closes #40

[erikzenker]

Pls rebase, then ready to merge :-)