Fix bug in Laguerre polynomial of negative order

It holds that:

  L_{n}(-x) = \exp{-x} L_{-n-1}(x)
            = \exp{-x} L_{-(n+1)}(x)

by using

  k := n + 1  =>  n = k - 1

it follows that:

  L_{k-1}(-x) = \exp{-x} L_{-k}(x)

and finally:

  L_{-k}(x) = \exp{x} L_{k-1}(-x)
Fix polynomials for fixed negative order

This:

  >>> laguerre(-3, 4)
  laguerre(-3, 4)

will no longer raise a:

  ValueError: The index n must be nonnegative integer (got -3)

but instead expand into:

  >>> expand_func(_)
  exp(4)*laguerre(2, -4)

and further:

  >>> expand_func(_)
  17*exp(4)