Add vapor fraction as option for setting state in YAML files

[speth]

• 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

Please fill in the issue number this pull request is fixing

Addressing a question from @ischoegl in on Cantera/cantera-website#91.

Changes proposed in this pull request

• Allows (T, Q), (P, Q), and (T, P, Q) as state definitions for pure fluid phases. For the last one, requires the three variables to be self-consistent.
• Fixes an issue that I ran into where leaving the pure-fluid-name out of the phase definition gave you a water object instead of throwing an exception.

[bryanwweber]

Any reason not to cover the error messages and the vapor-fraction option with a test?

[ischoegl]

Everything else looks good to me.

[codecov]

Codecov Report

Merging #784 into master will increase coverage by 0.02%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #784      +/-   ##
==========================================
+ Coverage   71.44%   71.46%   +0.02%     
==========================================
  Files         372      372              
  Lines       43952    43980      +28     
==========================================
+ Hits        31400    31430      +30     
+ Misses      12552    12550       -2

Impacted Files	Coverage Δ
include/cantera/thermo/ThermoPhase.h	28.28% <ø> (ø)	⬆️
src/thermo/PureFluidPhase.cpp	59.72% <ø> (+0.08%)	⬆️
src/tpx/utils.cpp	94.59% <0%> (ø)	⬆️
test/thermo/thermoFromYaml.cpp	100% <100%> (ø)	⬆️
src/thermo/ThermoPhase.cpp	69.59% <95.65%> (+0.92%)	⬆️
src/base/AnyMap.cpp	87.9% <0%> (+0.4%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4eb375e...f7c33c6. Read the comment docs.

[bryanwweber]

@speth I added one more test of the failure condition where the substance is unknown, to make sure that change isn't reverted. Feel free to drop it if you don't think it's necessary. This just seemed like the easiest way to add that change without a bunch of back and forth about what I meant 😄

[speth]

The new phase definition you added breaks the test python:test_composite.TestModels.test_load_thermo_models, because it tries to instantiate every phase defined in thermo-models.yaml. If you really want to have a test for this, I would suggest putting the phase definition string in-line, rather than creating yet another input file just for this.