updateSlot(intent, entity, mandatory, questions) {
    if (!this.intents[intent] || !this.intents[intent][entity]) {
      return this.addSlot(intent, entity, mandatory, questions);
    }
    const slot = this.intents[intent][entity];
    if (mandatory !== undefined) {
      // Update mandatory flag only if provided
      slot.mandatory = mandatory;
    }
    slot.locales = Object.assign(slot.locales, questions);
    return this.intents[intent][entity];
  }
Adds/modifies the parameter of a slot for a given intent and entity.
Slot questions for same locales as already existing will be overwritten.
If the slot for the intent and entity does not exist it fill be created.
@param {String} intent Name of the intent.
@param {String} entity Name of the entity.
@param {boolean} mandatory Flag indicating if is mandatory or optional.
@param {Object} [questions] Question to ask when is mandatory, by locale.
@returns {Object} New/Modified slot instance or undefined if not existing