Fix error messages about Picture being required even if it is included

This fix is for picture names that contain spaces in their id (ie "Picture 1", "picture 2" etc) ,causing  odt2Daisy to throw an error about an element named Picture being required even if it is in the document and has an id.

Seemed better to do it this way than trying to explain the reason picture ids must not have whitespace in them to the end user.

BTW ignore the first commit please as I realised the error of this way of doing things