Add an option that makes it possible to ensure that the last point is exactly for Tmax; make sure that we never exceed Tmax

For a 1-stage integrator ([`i ∈ {0}`, `c_[0] = 0`, symplectic Euler](http://en.wikipedia.org/wiki/Semi-implicit_Euler_method)) we would be missing the intrinsic acceleration in your example, but we are using a [6-stage method](https://github.com/mockingbirdnest/Principia/blob/master/integrators/symplectic_partitioned_runge_kutta_integrator_body.hpp#L21-38)!
One [easily](https://github.com/mockingbirdnest/Principia/blob/master/integrators/symplectic_partitioned_runge_kutta_integrator_body.hpp#L51-54) computes the nodes,
```mathematica
In[1] :=
Drop[Prepend[Accumulate[
   { 0.1193900292875672758,  0.6989273703824752308,
    -0.1713123582716007754,  0.4012695022513534480, 
     0.0107050818482359840, -0.0589796254980311632}], 0.], -1]

Out[1] = 
{0., 0.1193900292875672758, 0.818317399670042507,
 0.647005041398441731, 1.048274543649795179,
 1.058979625498031163}
```
With this method, the last 4 stages will use the intrinsic acceleration.

We also notice here that the intrinsic acceleration needs to be sampled after the end of the integration step, which is mildly annoying. We may want to switch to a set of coefficients that does not do that when the intrinsic acceleration is measured (the steps are `0.02 s` long in that case anyway, so 5th order is overkill). When we have a way to compute the intrinsic acceleration ourselves (predicted burns etc.) this is not a problem.