Genericise derivative generation

Hi,

By genericise them I mean to make it possible to call `deriveImageCopy()` on any copy that has a TIFF image.  That will be necessary for us while making co-masters.  Banjo decides whether to call `deriveImageCopy()` on the `MASTER_COPY` or the `COMASTER_COPY`, and Amber just does what it needs to, possibly overriding the previous `ACCESS_COPY`.

The methods that generate jp2s from tiffs are already pretty generic, but there was an error message that will be confusing if not fixed. ;-)

Huw.