improving special functions in GSoC 2017

[Colin Macdonald]

I added a rough blurb for a GSoC 2017 project:

http://wiki.octave.org/Summer_of_Code_Project_Ideas#Numerical

Motivation: I've seen various poor behaviour from our special functions 
over the last couple months.  I think someone mentioned improving all 
that would be a good GSoC project.  I agree.

Please link bugs there if you know of others.

Please also add your name as potential mentors: I'm not an expert on 
special functions, but I'm willing to help out.

Colin