merge_complementary_vectors
take several vectors with complementary entries
(i.e, vector of same length, that are such that only one vector has a non NA value
for each entry), and merge them into one.
merge_complementary_vectors(comb, mat)
row vector of the same size as entry matrix.
vector giving the rows to be kept.
matrix containing the vectors as rows.
The vectors are selected from the entry matrix by comb. At each entry, the vectors
are added using function add_complementary
.