add ability to define headers aliases for output files, add a new
parameter named headers_aliases for poincare plot;
this new parameter must correspond to statistics_names parameter; an
example of use:
statistics_names = 'sd1d, sd1a, sd2d, sd2a, sdnnd, sdnna, ca, cd, c1d,
c2d, c1a, c2a'
headers_aliases = 'sd1w, sd1s, sd2w, sd2s, sdnnw, sdnns, cs, cw, c1w,
c2w, c1s, c2s'