Convert the *sp code to C++?

[krivit]

Currently, changestats_dgw_sp.[hc] are a highly complex mess of nested macros, because they need to implement all 36 combinations of the following:

• 2 focus edge values: edgewise and dyadwise
• 6 two-path/shared partner configurations: UTP, OTP, ITP, OSP, ISP, RTP
• 3 calculation types: specific list of sp values, sp values from 1 to cutoff (with error if cutoff exceeded), and geometrically weighted

Now that there is a C++ term API, it would make sense to rewrite this into a C++ template.