Use make_shared in pricing engines

[jewonj0620]

This PR replaces old-style ext::shared_ptr<T>(new ...) constructions with ext::make_shared<T>(...) under ql/pricingengines.

The change is intended as a mechanical modernization and follows the existing ext::make_shared usage in the codebase.

In addition, added some auto declaration when rhs and lhs return types are identical

[coveralls]

coverage: 74.829% (-0.01%) from 74.843% — jewonj0620:feature/use-make-shared-pricingengines into lballabio:master

[lballabio]

As we're going through this modernization, I think it's fair to use auto even when lhs and rhs are not the same. It's probably more convenient to do this now than to search for all these cases later. What do you think?

[jewonj0620]

@lballabio

Okay. Sounds good to me. I will proceed to use auto instead of alias in declaration. I will apply the patch for this from this pr

Hope the review is not confused. will notify after the update!