implement Meijer G function

[rws]

This function would allow to express the result from differentiation of several special functions with respect to the order parameter, which at the moment gives an error:

sage: diff(exp_integral_e(n,x),n)
NotImplementedError: The derivative of this function is only implemented for 
sage: gamma_inc(n,x).diff(n)
D[0](gamma)(n, x)

sage: sympy.expint(n, x).diff(n)
-x**(n - 1)*meijerg(((), (1, 1)), ((0, 0, -n + 1), ()), x)
sage: sympy.uppergamma(n, x).diff(n)
meijerg(((), (1, 1)), ((0, 0, n), ()), x) + log(x)*uppergamma(n, x)
sage: sympy.lowergamma(n, x).diff(n)
-meijerg(((), (1, 1)), ((0, 0, n), ()), x) - log(x)*uppergamma(n, x) + gamma(n)*polygamma(0, n)
sage: meijerg?
Object `meijerg` not found.

sage: laplace(cos(-1/t), t, s, algorithm='sympy')
...
AttributeError: Unable to convert SymPy result (=meijerg(((), ()), ((-1/2, 0, 1/2), (0,)), s**2/16)/4) into Sage

Mpmath has numerics too.

​https://en.wikipedia.org/wiki/Meijer_G-function

[vdelecroix]

update milestone 8.3 -> 8.4