Return list of counts from list l of lengths.
Note
Counting starts at 1.
abjad> listtools.lengths_to_counts([1, 2, -3, -4, 5])
[[1], [2, 3], [-4, -5, -6], [-7, -8, -9, -10], [11, 12, 13, 14, 15]]
abjad> listtools.lengths_to_counts([1, 0, -3, -4, 5])
[[1], [], [-2, -3, -4], [-5, -6, -7, -8], [9, 10, 11, 12, 13]]