improves the accuracy near q=0.
This closes issue 1401.

This closes issue #1403 (did not terminate upon absence of GSL)

plot_diff script: compute change relative to average of old and new.

added swig preprocessor guards for certain overloaded operators due to swig's incomplete support for decltype

Test of precomputed factorial vs tgamma outcommented because tgamma is too unprecise under glibc 2.12. Instead, test 150! vs value from Wolfram Alpha.