default Locator Url
The base URL that should be used by the ReaderPublication when it creates new Locators.
NOTE: This option can only be set when calling ReadingSystemEngine.loadPublication(). It is NOT possible to change defaultLocatorUrl using ReaderPublication.setOptions().
If not specified, it will be generated based on the Publication's hash signature. (See IPublication.getHashSignature()) See the publication format implementation (EpubReaderPublication, PdfReaderPublication, etc) for details on how the defaultLocatorUrl is generated.