add support and validation for Graduate Software Accessory roms.

[cormacj]

Graduate Software CPM+ Accessory ROMs don't use a standard CPC rom format and caprice was rejecting these.

This change extends the ROM loading code to validate these ROMs and load if it passes the checks.

Reference: https://www.cpcwiki.eu/index.php/Graduate_Software

[ColinPitrat]

Thanks!