Handle non-standard SEG-Y metadata created by the Headwave SEG-Y importer.

The Bluware Headwave product predates the VDS specification and we have a lot of data that has non-standard metadata that stores converted SEG-Y text (converted to ASCII) and binary (converted to little-endian) headers. In order to make testing easier this change adds support for these non-standard metadata to the SEGYExport tool.