adding convenience operators about fastad HOT 3 CLOSED

Hey, thanks for trying out my library!

So for reverse AD, we don't actually compute anything until we call autodiff on the final expression. When we write for example

Var<double> a, b{2};
a = 5. * b * b;

the result of 5. * b * b does not get computed and "stored" in a. The expression a = 5. * b * b returns a completely different object representing it. So a working example would be something like

Var<double> a, b{2};
auto expr = (a = 5. * b * b);
autodiff(expr); // now a and b contain derivative values

And I couldn't think of a good use-case where operator+= would shine from simply using operator=. Is there a use-case you had in mind?

For forward AD using ad::ForwardVar, I think it makes sense to overload these operators so I can add that feature.

For allowing floating point values with ad::Var, I think that's a fair point too, I'll get around adding that! For now, you have to use ad::constant(5.) in place of 5..

from fastad.

#84 literals can now be used to construct expressions without the need for user to explicitly write ad::constant

from fastad.

#86 includes support for convenience operators: operator+=, operator-=, operator*=., operator/= for reverse mode

from fastad.