Draft: Rename collective calls

[TheAssembler1]

Rename all client API collective calls to be suffixed with _coll.

Locally compilation with and without PDC_ENABLE_MPI succeeds. Also the serial and shared tests work as expected locally.

#288

[jeanbez]

Can we unify if we pass the MPI communicator as well?

[TheAssembler1]

I added the communicator to the Client API calls. This required some of the tests to change since some of the serial tests were calling collective calls which would previously work on those that did not require the MPI communicator. I've added conditional macros to call the collective/independent version depending on if ENABLE_MPI is set.

[TheAssembler1]

Also, previously, the ENABLE_MPI was set in in the CMakeLists.txt using set. However, this was causing some files to have ENABLE_MPI defined and others would not have it defined. I fixed this by using add_compile_definitions(ENABLE_MPI=1) instead. I'm not sure if this is the correct fix...

[TheAssembler1]

Some of the collective private client PDC functions use a global variable PDC_CLIENT_COMM_WORLD_g which is set to the communicator instead of an extra comm parameter. I left those as is and only adjusted the public client API functions.