Freeze version for diffpy.Structure.
freeze version for diffpy.utils.
Fix check for presence of tag on a git branch.
Freeze versions for pyobjcryst, periodictable, cxxtest.
Version freeze for libdiffpy, diffpy.srreal, diffpy.srfit.
Adjust for the removed buildtools directory.
Freeze DANSE SANS codes version.
Merge parallel changes in branch 'master'.

src-addons should not be included with the package.
This reverts ae2deeb..efd47ef.