function registerTag(tag, name, type = undefined, isArray = false) {
  fieldTags[name] = tag;
  fieldTagNames[tag] = name;

  if (type) {
    fieldTypes[name] = type;
  }
  if (isArray) {
    arrayFields.push(tag);
  }
}
Registers a new field tag
@param {number} tag the numeric tiff tag
@param {string} name the name of the tag that will be reported in the IFD
@param {number} type the tags data type
@param {Boolean} isArray whether the tag is an array