This function turns an integer specifying an order of a derivative into the
equivalent linear differential operator object. It is also useful for
checking that an object is of the "Lfd" class.
USAGE:
int2Lfd(m=0)
OPTIONAL ARGUMENTS:
m
either a nonnegative integer or a linear differential operator object.
VALUE:
e linear differential operator object of the "Lfd" class that is
equivalent to the integer argument.