Add dilution options to equivalence ratio functions

[g3bk47]

Two changes are proposed in this PR:

1. add a new argument to equivalence_ratio to allow users to ignore certain species, e.g. as a result of dilution with an inert species
2. add two new arguments to set_equivalence_ratio to dilute a mixture after the mixture composition based on equivalence ratio has been determined.

All changes are backward compatible. Discussion on these changes can be found here:
Cantera/enhancements#108
#521
https://www.google.com/search?q=cantera+users+group&rlz=1C1GCEB_enDE918DE918&oq=canter&aqs=chrome.0.69i59l3j69i57j69i60l3j69i65.6761j0j7&sourceid=chrome&ie=UTF-8

The first change is used like this:
gas.equivalence_ratio(fuel="H2", oxidizer="O2:0.21,N2:0.79", include_species=["H2","O2"])
This computes the equivalence ratio as before, but ignores all species except H2 and O2

The second change is used like this:
gas.set_equivalence_ratio(2.0, "H2", "O2:0.21,N2:0.79", fraction="diluant:0.1", diluant="H2O")
This first creates a mixture of H2 and air with equivalence ratio 2 and then dilutes it with H2O so that the mole fraction of the diluant in the final mixture is 0.1. Similar options exist to fix the fuel and oxygen fraction in the final mixture.

Comprehensive descriptions have been added to the examples in interfaces/cython/cantera/examples/thermo/equivalenceRatio.py to showcase these additions to Cantera.

[ischoegl]

Hi @g3bk47 ... thank you for the PR! I believe this is a nice addition that addresses Cantera/enhancements#108 as well as some recent discussions on the user forum.

While there may be another round of suggestions, I mainly focused on the code for the time being. Cantera has a couple of built-in capabilities that are not obvious, but should help simplify some of what you implemented. Also, any new code addition should follow some style conventions more closely: e.g. empty spaces left and right of comparisons and most operators, double quotes for Python strings (whenever possible), etc.

I appreciate that you already added comprehensive unit tests, and updated the example. I honestly haven't checked the actual implementation yet, but it looks like you have most of the bases covered ...

[ischoegl]

@g3bk47 ... thank you for the fast turn-around! While I believe that the implementation is sound, I have some additional requests (which are mostly formatting).

[codecov]

Codecov Report

Merging #1206 (da8e2d0) into main (041559d) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1206   +/-   ##
=======================================
  Coverage   65.41%   65.41%           
=======================================
  Files         318      318           
  Lines       46085    46085           
  Branches    19604    19604           
=======================================
+ Hits        30145    30147    +2     
+ Misses      13426    13424    -2     
  Partials     2514     2514           

Impacted Files	Coverage Δ
src/base/AnyMap.cpp	85.29% <0.00%> (+0.18%)	⬆️

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[ischoegl]

@g3bk47 … thank you for taking care of all the requested changes! I’ll approve, but leave it open for another day before merging in case anyone else wanted to comment.