Implement the MultiStateWeightProvider and refactor the WeightProvider

[GiulioRomualdi]

The MultiStateWeightProvider is a provider that will allow us to perform gain scheduling. The user should provide a configuration file containing the name and the weight for each state. Then the provider will interpolate among the states using a FirstOrder stable system. The settling time of the system should be set in the configuration file.

I also implemented the associated tests and the python bindings.

I also add the possibility to set the weight provider directly in the ILinearTaskSolver::addTask() method.

The PR also introduces the concept of output and input ports. These two interfaces contain functions to get and set signals only. And advanceable inherits from them. Thanks to this w can split the concept of advanceable from the concept of getting values. For instance

class Foo : public Advanceable<double> {...};

void f(const OutputPort<double>& port);


int main() {
    Foo foo;
    f(foo);

Now the user knows that f will only get the output of the advanceable and nothing more, (for instance it will never call advance or setInput)

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This substitutes #507

[S-Dafarra]

Mostly comments about documentation

[GiulioRomualdi]

@S-Dafarra I think I handled all your comments

[GiulioRomualdi]

brew fails as already reported in: robotology/robotology-superbuild#1234