计算离散时间信号间卷积和，并确定卷积信号的起止时刻点
信号处理卷积例程(convolution routine for signal processing
% --------------------------------------------------
% [y,ny] = conv_m(x,nx,h,nh)
%　y = convolution result
% ny = support of y
%　x = first signal on support nx
% nx = support of x
%　h = second signal on support nh
% nh = support of h)