The dump data function

def dump_data(y, dt):
    # write out 2-column files with t and y[name] for each name:
    for name in y.keys():
        ofile = open(name+'.dat', 'w')
        for k in range(len(y[name])):
            ofile.write('%12g %12.5e\n' % (k*dt, y[name][k]))
        ofile.close()