Hotfix and code factorization

• Factorize the conj/id functions in spm2dense and spmPrint
• Add comments to subfunctions in spm2dense and spmPrint
• Fix minor issues reported by coverity
• Add frobenius_merge functions from pastix
• Fix issue with spmExpand in the distributed case
• Factorize the output strings declarations in the testings

This PR contains preliminary changes toward !25 (merged)