Bugfix for crash during gtSetupReconstruction:

type of parameters.rec.method was set to 'double' instead of 'char' which
resulted in an error. gtModifyStructure showed empty field and didn't
save the given method, either 3DART or 2DFBP, therefore crashing at
gtAstraAbsorption because no method was given.


Signed-off-by: Stefan Schmiederer <sschmied@esrf.fr>