getCacheKey (params) {
    if (this.options.cache === true) {
      const cacheProperties = this.cacheKeyMethod(this.id, params)
      const hash = Fact.hashFromObject(cacheProperties)
      return hash
    }
  }
Generates the fact's cache key(MD5 string)
Returns nothing if the fact's caching has been disabled
@param  {object} params - parameters that would be passed to the computation method
@return {string} cache key