Rename parsec_add2arena

[abouteiller]

Original report by me.

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This name does not make sense anymore, we should simplify it to something less weird/internal looking as it is part of the user-facing interface.

[bosilca]

• parsec_add2arena: parsec_arena_datatype_set_matrix_tile (or adt instead od arena_datatype). This function should internally call parsec_arena_datatype_construct
• parsec_del2arena: should be removed, and all calls replaced with parsec_type_free