Returns an array of aligned data from a list of data arrays.
Data is the concatenation of rows, the zero-th column being the alignment
value (or index), the subsequent ones the aligned data. The size of the
resulting array will be of size 2*N + 1 where N is the number of data sets
to be aligned. Rows are sorted: the first data set will be in column
1 and N+1, the second in 2 and N+2. If still valid or if none are available
at some point, data obtained in previous rows are repeated in the current
one.