small speedup in PeriodicBasis::intersect_supports()
small speedup in PeriodicBasis::intersect_supports()

kedingagnumerikunimarburg/Marburg_Software_Library,agnumerikunimarburg/Marburg_Software_Library,wavelet-and-multiscale-library/Marburg_Software_Library,wavelet-and-multiscale-library/Marburg_Software_Library