gamma_inc_inv fails for low precision floats

[Andreas Noack]

See the error below. The problem is that we currently promote the arguments to Float64 and then promote back but generally Float64(p) + Float64(p) != 1 when p + q == 1 if p and q have lower precision than Float64 and that will trigger an exception.

julia> gamma_inc_inv(1.0f0, 0.9f0, 0.1f0)
ERROR: ArgumentError: p + q must equal one but is 0.9999999776482582
Stacktrace:
 [1] _gamma_inc_inv(a::Float64, p::Float64, q::Float64)
   @ SpecialFunctions ~/.julia/dev/SpecialFunctions/src/gamma_inc.jl:927
 [2] _gamma_inc_inv
   @ ~/.julia/dev/SpecialFunctions/src/gamma_inc.jl:1017 [inlined]
 [3] gamma_inc_inv(a::Float32, p::Float32, q::Float32)
   @ SpecialFunctions ~/.julia/dev/SpecialFunctions/src/gamma_inc.jl:922
 [4] top-level scope
   @ REPL[155]:1

Maybe the solution is to change the Float16 and Float32 methods to compute 1 - Float64(min(p, q)) and verify that this number is identical to the input in the original precision before calling the Float64 method.