Create forward prjection structure for twin cluster.

Add functions to create forward projection structure for twin cluster.

Signed-off-by: Zheheng Liu zheheng.liu@esrf.fr

Closes #2 (closed)

requested review from @vigano

assigned to @zliu

approved this merge request

added 4 commits

• a73ea180 - 1 commit from branch master
• fc1d0109 - create forward prjections for twin cluster.
• 73b5cb46 - some changes for gtGrainMerge
• eaced501 - Add noise variances to cluster forward projection structure

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