Implement checkpointing API in C++ and Fortran

[maarten-ic]

The 0.6.0 release added support for checkpointing to the Python API. The same should be exposed to the C++ and Fortran APIs as well.

Task list (no dependencies):

• [x] Port new internal classes to C++
  • [x] libmuscle.api_guard
  • [x] libmuscle.checkpoint_triggers
  • [x] libmuscle.snapshot
  • [x] libmuscle.snapshot_manager
  • [x] MMPClient and Communicator functions
• [x] Create unit tests for ported internal classes
  • [x] libmuscle.api_guard
  • [x] libmuscle.checkpoint_triggers
  • [x] libmuscle.snapshot
  • [x] libmuscle.snapshot_manager
  • [x] MMPClient and Communicator mocks

Task list, dependent on merging the profiling branch:

• [x] Implement InstanceFlags (PR #195)
  • [x] C++
  • [x] Fortran bindings
  • [x] Write tests for defined constants
• [x] Implement new calls to the muscle manager in MMPClient
• [x] Implement getting/setting port counts in Communicator

Task list, blocked by PR #195

• [x] Implement public API on Instance
  • [x] C++
  • [x] Fortran bindings
• [x] Add integration tests for checkpointing C++ and Fortran code
• [x] Documentation
  • [x] Add C++ and Fortran examples for checkpointing in reaction-diffusion
  • [x] Update checkpointing.rst to link to the C++ and Fortran examples
  • [x] Update checkpointing.rst with C++ and Fortran code samples