[MATLAB] Deprecate IdealGasMix function

[bryanwweber]

The Solution function provides the same utility for creating Solutions.
Conversion to YAML format files should be done using the command line
tool.

Checklist

• There is a clear use-case for this code change
• The commit message has a short title & references relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• The pull request is ready for review

[speth]

I'm 👍 on deprecating this function.

I think the docstring needs to be updated to indicate the deprecation as well.

[codecov]

Codecov Report

Merging #806 into master will decrease coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #806      +/-   ##
==========================================
- Coverage   71.54%   71.39%   -0.16%     
==========================================
  Files         372      372              
  Lines       44348    43482     -866     
==========================================
- Hits        31730    31045     -685     
+ Misses      12618    12437     -181

Impacted Files	Coverage Δ
include/cantera/thermo/PDSS.h	0% <0%> (-100%)	⬇️
include/cantera/equil/vcs_MultiPhaseEquil.h	0% <0%> (-100%)	⬇️
include/cantera/equil/MultiPhaseEquil.h	0% <0%> (-95%)	⬇️
include/cantera/thermo/MolalityVPSSTP.h	20% <0%> (-80%)	⬇️
include/cantera/transport/GasTransport.h	20% <0%> (-80%)	⬇️
include/cantera/thermo/SpeciesThermoInterpType.h	20% <0%> (-70%)	⬇️
include/cantera/base/global.h	38.46% <0%> (-54.88%)	⬇️
include/cantera/thermo/VPStandardStateTP.h	20% <0%> (-51.43%)	⬇️
include/cantera/thermo/Phase.h	50% <0%> (-36.05%)	⬇️
src/clib/Cabinet.h	0% <0%> (-26.42%)	⬇️
... and 60 more

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