getThangType (slug) {
    const thangType = this.loadedThangTypes.get(slug)
    if (!thangType) {
      console.error(`Make sure '${slug}' thangType is loaded before getting`)
      return
    }
    return thangType
  }
Get a loaded thangType by slug.
@param {string} slug - Slug of a thangType
@returns {object|undefined} May return thangType or undefined