gamma_inc doesn't handle NaNs

[Andreas Noack]

They currently cause infinite loops. E.g.

gamma_inc(NaN, 0.3)

hangs in

SpecialFunctions.jl/src/gamma_inc.jl

Lines 453 to 461 in d829f0a

 while true 
    l += 1.0 
    c *= -x/l 
    t = c/(a + l) 
    sm += t 
    if abs(t) <= tol 
        break 
    end 
 end 

. I think that Morris' old fortran code just didn't handle NaNs at all. Should we just propagate the NaNs or should we throw in the same way as we do for other illegal input such as in

SpecialFunctions.jl/src/gamma_inc.jl

Lines 796 to 799 in d829f0a

 if a < 0.0 || x < 0.0 
     throw(DomainError((a, x, ind), "`a` and `x` must be greater than 0 ---- Domain : (0, Inf)")) 
 elseif a == 0.0 && x == 0.0 
     throw(DomainError((a, x, ind), "`a` and `x` must be greater than 0 ---- Domain : (0, Inf)"))