C++17 Mathematical Special Functions

[mborland]

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

• assoc_laguerre #404
• assoc_legendre #445
• beta #648
• comp_ellint_1 #463
• comp_ellint_2 #664
• comp_ellint_3
• cyl_bessel_i #600
• cyl_bessel_j #601
• cyl_bessel_k #602
• cyl_neumann #603
• ellint_1 #462
• ellint_2 #663
• ellint_3
• expint #650
• hermite #400
• legendre #444
• laguerre #402
• reimann_zeta #547
• sph_bessel #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