- changed function::diff() to be more tolerant by checking first if the

- changed function::diff() to be more tolerant by checking first if the
  nth argument when differentiated is non-zero and *then* building the sum.
- added support for overloaded polygamma functions psi(x) and psi(n,x).
- changed return code of atan2_diff to be somewhat simpler and adjusted
  check/differentiation.cpp to account for this.