Do not require the invocation of ::get() in Factory

Factory is a singleton. Currently, we access the unique instance with a static method get(). After that, we call other methods:

FactorySensor::get().registerCreator(...);
FactorySensor::get().create(...);
[...]

This issue proposes a cleaner API, as follows:

FactorySensor::registerCreator(...);
FactorySensor::create(...);
[...]

The idea is to make get() private, and registerCreator(), unregisterCreator(), create() static, which would internally invoke get() to get access to the factory:

For example, the method create() would be:

template<class TypeBase, typename... TypeInput>
class Factory
{
  static TypeBasePtr create(const std::string& _type, TypeInput... _input);
  {
    typename CallbackMap::const_iterator creator_callback_it = get().callbacks_.find(_type); // <-- use get() here

    if (creator_callback_it == get().callbacks_.end())                                       // <-- use get() here
        // not found
        throw std::runtime_error(get().getClass()                                            // <-- use get() here
                                 + " : Unknown type \"" 
                                 + _type 
                                 + "\". Possibly you tried to use an unregistered creator.");

    // Invoke the creation function
    return (creator_callback_it->second)(std::forward<TypeInput>(_input)...);
  }
}