[refactor] usage of UT_icd in nrm_vector_s

[iddaoudi]

UT_array already contains UT_icd (see include/internal/utarray.h).
Yet in the definition of the nrm_vector structure (see src/utils/vectors.c), we define UT_array AND UT_icd separately, and then use them as a->array and a->icd.
Code should be refactored to only use UT_array in the definition of nrm_vector and then access the icd via a->array.icd.