Need systematic way to evaluate integrals, limits, sums, ...

[kpeeters]

At present expressions containing integrals, limits, sums and so on can be evaluated by feeding them through simplify or using map_sympy, e.g.

 ex := \int{x**2}{x,0,1};
 simplify(ex);

However, that's overkill, and it also makes it harder to replace this with Cadabra's own functionality at some later time.

Using evaluate could work, but perhaps expanding it out into evaluate_limits, evaluate_components, evaluate_integrals and so on would be better.