Add diffrules for log1pmx and logmxp1

[simsurace]

This is supposed to fix JuliaStats/LogExpFunctions.jl#44.

There may be more optimal orders of operations for these derivatives.
Also, could someone assist me in what would be mandatory tests for these additions?

[simsurace]

Looks like JuliaStats/LogExpFunctions.jl#45 is actually a prerequisite.

[devmotion]

It's already tested automatically but maybe it's helpful to adjust the domain of inputs (both for the primal and the finite diff comparisons), similar to some other functions in runtests.jl. I don't remember the default domain by heart, so maybe it's not needed at all.

Ironically, tests fail because the functions are not defined for Float32 (yet). So for the time being, probably we should skip Float32 tests for these inputs (or check that they throw an error, to be notified when this changes).

[codecov-commenter]

Codecov Report

Merging #82 (b1795d3) into master (7dde134) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   96.96%   97.00%   +0.03%     
==========================================
  Files           3        3              
  Lines         165      167       +2     
==========================================
+ Hits          160      162       +2     
  Misses          5        5              

Impacted Files	Coverage Δ
src/rules.jl	100.00% <100.00%> (ø)

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[devmotion]

Thanks, looks good! I just have one suggestion to make finite differencing even less likely to fail.

[devmotion]

Looks good to me, thank you!

I'll merge and release once tests pass (and feel free to ping me if I forget it 🙂).

[devmotion]

The MTK test errors are unrelated.