Quadratization under maximization

[pedromxavier]

Currently, PBO assumes the minimization sense throughout its methods.
This has a special impact in the quadratization where different methods are selected according the each term's sign.
As pointed out in JuliaQUBO/ToQUBO.jl#77, this gets in the way of efficient, in-place quadratization.
A solution would be to add an optional flag or sign (-1, 1) parameter to multiply and divide each term`s coefficient before and after quadratization, respectively.

Any input, @bernalde @pedroripper?

[bernalde]

I support the use of the optional flag parameter (we use it in other projects as well). Just make sure to keep the "books" for the sign inversion. Tests will be required.

[pedroripper]

The optional flag sounds good to me too