calculate (params, almanac) {
    // if constant fact w/set value, return immediately
    if (Object.prototype.hasOwnProperty.call(this, 'value')) {
      return this.value
    }
    return this.calculationMethod(params, almanac)
  }
Return the fact value, based on provided parameters
@param  {object} params
@param  {Almanac} almanac
@return {any} calculation method results