gtSetupReconstruction : fix bug if absorption volume already exists but...

gtSetupReconstruction : fix bug if absorption volume already exists but absorption mask doesn't exist.

Signed-off-by: Laura Nervo <laura.nervo@esrf.fr>

git-svn-id: https://svn.code.sf.net/p/dct/code/trunk@485 4c865b51-4357-4376-afb4-474e03ccb993