Plot sets of vectors starting at the origin

USAGE
plot_vectors(vectors)
plot_vectors(vectors,normalize)

INPUT
- VECTORS: cell array with Vix2 or Vix3 matrices (i=1,2,...) in the cell i, with the x-, y- and z-coordinates for Vi vectors
- NORMALIZE: 1 (normalize the vectors) or 0 (don't normalize)

OUTPUT
Figure with the vectors from VECTORS{i} starting at the origin. Colors for VECTORS{i}: blue (i=1,8,...), red (i=2,9,...), green (i=3,10,...), cyan (i=4,11,...), magenta (i=5,12,...), yellow (i=6,13,...), black (i=7,14,...).