Fix compilation warnings

[ischoegl]

Changes proposed in this pull request

Fix compilation warnings caused by comparisons of int and size_t.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review

[codecov]

Codecov Report

Merging #1346 (8ed4a6f) into main (ade1a38) will increase coverage by 0.00%.
The diff coverage is 81.81%.

@@           Coverage Diff           @@
##             main    #1346   +/-   ##
=======================================
  Coverage   68.15%   68.15%           
=======================================
  Files         327      327           
  Lines       42658    42659    +1     
  Branches    17180    17180           
=======================================
+ Hits        29074    29075    +1     
  Misses      11291    11291           
  Partials     2293     2293           

Impacted Files	Coverage Δ
...e/cantera/zeroD/IdealGasConstPressureMoleReactor.h	100.00% <ø> (ø)
include/cantera/zeroD/MoleReactor.h	11.11% <ø> (ø)
src/thermo/Elements.cpp	88.00% <80.00%> (ø)
include/cantera/numerics/PreconditionerBase.h	25.92% <100.00%> (+2.84%)	⬆️

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[ischoegl]

Alright - I fixed the warnings per discussion (together with two more I located, notably the mildly annoying NPY_1_7_API_VERSION warning generated by Cython).

Otherwise, I believe this is good to go from my side.

[speth]

Thanks, @ischoegl. It's great to see that there is finally a fix for the Numpy/Cython deprecation warning, especially after #1334 made several more instances of it appear.

I just had one minor concern related to the changes to .gitignore.

[ischoegl]

@speth ... thanks for the review! I dropped the .gitignore bit, so this should be good to go.