Philippe 23/10/2019: OpenACC: rename _OPENACC key to MNH_OPENACC (_OPENACC is a standard OpenACC macro)

Philippe 03/10/2019: OpenACC: replace 'declare present' by 'data present' directives for assumed-size dummy arrays (to be standard compliant)

Philippe 02/10/2019: OpenACC: remove declare present directives from module declaration sections (to be standard compliant)