Check result of downloading data and adapt tests (CRAN requirement)

data are downloaded for vignette and tests.

CRAN now requires that the access to internet resources is checked :
'Packages which use Internet resources should fail gracefully with an informative message
if the resource is not available or has changed (and not give a check warning nor error).'

see what was done in SticsRFiles : branches 'fix-download_data' and 'fix-tests-download'