Merge branch 'spmSort' into 'master'

CMakeLists.txt

src/common.h

src/spm.c

src/spm_gather.c

src/z_spm.c

src/z_spm_expand.c

src/z_spm_sort.c

tests/CMakeLists.txt

tests/spm_dist_sort_tests.c

tests/spm_dof_sort_tests.c

tests/spm_tests.h

tests/z_spm_sort_tests.c