Expand BondFunctions::yield to accept dirty price as input too

[igitur]

Currently BondFunctions::yield() accepts only the clean price to solve for the yield. This PR adds a parameter to specify whether the input price is either the dirty or clean price.

[lballabio]

I'd like the extra parameter to be some kind of enumeration instead of a bool, but that might require some more thought (ideally, I'd extract Callability::Price or part of it and put it in scope of class Bond instead). In the meantime, the boolean can do.

[igitur]

Oh, yes, I absolutely agree. I never new about the Callability::Price enumeration. In fact, I would then also recommend to change the BondHelper (and subclasses) constructor to use that enumeration instead of the last useCleanPrice parameter and mark the current one as obsolete.

[igitur]

Thanks @lballabio . What is your opinion about breaking changes, i.e. replacing the useCleanPrice parameter in BondHelper with a similar priceType variable? Or should we create yet another overloaded constructor?

[lballabio]

I would add another constructor and deprecate the existing.