Fix support for ASA_WVI_1P

I've fixed some errors regarding the file structure of the product ASA_WVI_1P (bug [EPR-10] on the bug tracker, http://www.brockmann-consult.de/beam-jira/browse/EPR-10). There are also some modifications allowing to open and read files without bands (like ASA_WVI_1P files) and files with dataset descriptors referencing external datasets (not attached to the file).