Fix handling of reference pressure in MultiSpeciesThermo

[speth]

Changes proposed in this pull request

• Require all species in a phase to have the same reference pressure.
• Set the reference pressure for the phase based on the first species added (instead of ignoring the value specified by the species)

If applicable, fill in the issue number this pull request is fixing

Closes #1435

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 #1503 (a0f2fa4) into main (e2cdb1c) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##             main    #1503   +/-   ##
=======================================
  Coverage   69.77%   69.77%           
=======================================
  Files         377      377           
  Lines       58011    58017    +6     
  Branches    20695    20701    +6     
=======================================
+ Hits        40479    40484    +5     
- Misses      14577    14578    +1     
  Partials     2955     2955           

Impacted Files	Coverage Δ
include/cantera/thermo/MultiSpeciesThermo.h	100.00% <ø> (ø)
src/thermo/MultiSpeciesThermo.cpp	60.65% <83.33%> (+1.17%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[ischoegl]

@speth - thanks! Changes all look good to me.