calculate slope

def delta(var, key, tusec=None):
    '''calculate slope'''
    global last_delta
    if tusec is not None:
        tnow = tusec * 1.0e-6
    else:
        import mavutil
        tnow = mavutil.mavfile_global.timestamp
    dv = 0
    ret = 0
    if key in last_delta:
        (last_v, last_t, last_ret) = last_delta[key]
        if last_t == tnow:
            return last_ret
        if tnow == last_t:
            ret = 0
        else:
            ret = (var - last_v) / (tnow - last_t)
    last_delta[key] = (var, tnow, ret)
    return ret