yarp::os::Value constructors should be explicit

[drdanz]

These constructors

    Value(int x, bool isVocab = false);
    Value(double x);

allow to do evil things, I believe they should be explicit, forcing the user to call Value constructor when assigning stuff.

Unfortunately this is a breaking change, and I believe that this is used in several places outside YARP (and in YARP unit tests).

[pattacini]

I'm in favor of making them explicit!

[drdanz]

Will be fixed in https://github.com/robotology/yarp/pull/1677

[drdanz]

Fixed in https://github.com/robotology/yarp/pull/1677