Change physical and logical layout of the code

Change the file and folder layout, adding inline namespaces so the logical
namespaces match the physical locations of the files.
This also improves the organization and presentation of Doxygen documentation.

Change log:
* general:
    - Change names of classes and files to Snake case, starting with a capital
      letter
    - Tidy documentation and add @briefs everywhere
    - Adding namespaces inside #ifdef __cplusplus pragmas to C-files, such that
      the members will appear at the correct location in the documentation
* flow:
    - Move most of the classes in the flow namespace to a detail namespace
* piol:
    - Move communicator to utils namespace and rename CommunicatorInterface.hh
      to just Communicator.hh
    - Move Read and Write classes to namespace file and rename to Input_file and
      Output_file
    - Move Object read/Write-classes to detail namespace inside namespace file
    - Move ExSeis[PIOL] to configuration namespace
    - Rename Binary_file to IO_driver and move to own namespace
    - Create metadata namespace with Trace_metadata_key (formerly Meta) and a
      rules namespace containing everything concerning the rules for SEGY files
    - Create namespace segy for utilities and Trace header offsets (formerly Tr)
    - Create sort namespace inside operations namespace for the sort operations
* utils:
    - Move Distributed_vector classes to namespace of the same name
    - Remove mpi namespace and create mpi_utils for utilities and move
      MPI_Distributed_vector to distributed_vector namespace and MPI_types to
      types namespace
    - Create types namespace for various kinds of type definitions