[with patch, needs review] make gauss_sum() work for dirichlet characters when the base ring is CC

[bober]

This is a pretty small change. We add three lines to gauss_sum_numerical() to make it work when the base ring is a complex field, and change gauss_sum() to call gauss_sum_numerical() when the base ring is a complex field.

Note that it is actually much faster to compute the (approximate) gauss sum when the base ring is CC, as compared to when the base ring is a cyclotomic field.

sage: G = DirichletGroup(2981)
sage: chi = G.0
sage: timeit('chi.gauss_sum_numerical()')
5 loops, best of 3: 1.82 s per loop
sage: G = DirichletGroup(2981, ComplexField(200))
sage: chi = G.0                                  
sage: timeit('chi.gauss_sum_numerical()')        
25 loops, best of 3: 23.5 ms per loop

[roed]

Should this be marked as needs review?