C++17 Mathematical Special Functions

[mborland]

These likely can be adapted from the ones in Boost.Math but we should include them for completeness:

• [x] #404
• [x] #445
• [x] #648
• [x] #463
• [x] #664
• [ ] comp_ellint_3
• [ ] #600
• [ ] #601
• [ ] #602
• [ ] #603
• [x] #462
• [x] #663
• [ ] ellint_3
• [ ] #650
• [x] #400
• [x] #444
• [x] #402
• [x] #547
• [ ] #604
• [ ] sph_legendre
• [ ] sph_neumann

[ckormanyos]

I'll handle Riemann-zeta and the collection of Bessels first. I#ve been down those roads before.

[ckormanyos]

Ok ellint_1 and 2 are preliminarily done