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)

Philippe + Maxime MOGE 13/09/2016: imported + adapted gradient_v.f90 and gradient_w.f90 from Maxime's work