Allow solvers to return "Complex" roots

Some solvers in o.a.c.m.analysis.solvers are able to return the set of all complex roots of a function (namely "LaguerreSolver").
However, the functionality does not fit with the overall design of the classes in this package which focus of computing one real ("double") root within a user-defined interval.
An idea is to provide a "ComplexSolver" public interface whose "solve" method would return a "Complex[]" containing the complex roots.

Moreover some time ago, we decided to modify the Commons Math API so as to provide the "fluent API" idiom.