Move ipmpar data into static int array
Move cdflib_dpmpar into static inline

Point is to change the internal/external signature momentarily.
Lookup ipmpar from static array
Use internal dpmpar
Restore external API for {d,i}pmpar
Add Welch test using cdflib
Improve test case

Checks things more towards the center of the t distribution
where CDFLIB does a better job.