Add mutating of p fields for integrator

[Myrddinlefou]

In the integrator doc you present 3 mutating possibility : set_t, set_u and set_ut.

In my case I want to change the differential equation, and that can be realised by change the fiels p.

A new function name set_p! can probably do that, or there are an other way to do it ?

Thanks a lot for your work.

[ChrisRackauckas]

integrator.p = .... We are missing a setter function, but it's kind of different because unlike u or t, changing p doesn't effect the system's internal components.

[Myrddinlefou]

Yes, but how can I change p simply during an interrupt by integrator?