Check fix of alchimia

[dcesari]

An application with a complex list of derived variables (https://github.com/ARPA-SIMC/prodsim/blob/master/src/prodsim_thunderstorm_index.F90) generates an invalid grib id eccodes error in this line

libsim/volgrid6d/volgrid6d_alchimia_class.F03

Line 220 in 2ea7560

CALL copy(vg6dout%gaid(ilevel,itime,itimerange,ivarin), &

corrected by the IF (ivarin /= ivarout) check (moreover the error appears only with -On optimized code). More checks are needed on the feasibility of this situation ivarin == ivarout or its dependency on a (compiler?) bug.