Remove large blocks of duplicated code by calling morse_desc_init_user() from morse_desc_init() and morse_desc_init_diag()

instead of introducing RUNTIME_distributed_barrier

MORSE_Distributed_size, MORSE_Distributed_rank so that applications do not
hardcode the use of MPI.

Introduce RUNTIME_distributed_rank, RUNTIME_distributed_size,
RUNTIME_distributed_barrier, so that MORSE does not hardcode the use of MPI
either.

This allows to use simgrid-mpi.

in MORSE_Desc_Create_User we can use the matrix in mat field without allocating it --> avoid to free it in this case, users could need it and will manage deallocation

change the way we include our own header files --> relative to the root - when plasma is in the same env, chameleon can take some headers not belonging to it (ex: #include descriptor.h, this file states in plasma install dir also) which make compilation errors

do not allocate a matrix if NULL pointer is given for mat in MORSE_Desc_Create_User --> let the user manage the allocation of his data

change copyright - correct whitespace - place cmake module depending on chameleon in cmake_modules and no more in cmake_modules/morse