Refactor gamma_inc_inv

[devmotion]

Refactors gamma_inc_inv as suggested in #374 (review):

• Checks of p + q == 1 and handling of special cases (iszero(p) and iszero(q)) are performed without promotions
• Internally, computations are only performed with min(p, q)
  (the PR does not compute 1 - min(p, q) as in Handle the case when Float64(p) + Float64(q) != 1 in gamma_inc_inv #374)

Moreover, the PR

• gets rid of redundant computations of logr and log(p)
• fixes promotion of arguments of gamma_inc_inv

[codecov]

Codecov Report

Merging #377 (b8f3057) into master (9dd5bf8) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #377      +/-   ##
==========================================
- Coverage   92.34%   92.32%   -0.03%     
==========================================
  Files          12       12              
  Lines        2797     2789       -8     
==========================================
- Hits         2583     2575       -8     
  Misses        214      214              

Flag	Coverage Δ
unittests	92.32% <100.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/gamma_inc.jl	91.75% <100.00%> (-0.11%)	⬇️

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[giordano]

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