SurfPhase does not bound chemical potentials at zero mole fraction

[speth]

Problem description

The implementation of chemicalPotentials in SurfPhase does not bound the chemical potentials to finite values at the limit of zero mole fraction for each species. This is different from the behavior of other phase models, and inconsistent with the definition of the partialMolarEntropies method for this phase.

>>> import cantera as ct
>>> s = ct.Interface('test/data/surface-phases.yaml', 'Pt-surf')
>>> s.TPX = 300, ct.one_atm, 'Pt(s): 0.7, H(s): 0.3, O(s): 0'
>>> s.gibbs_mole
-12463347.594169196

>>> s.chemical_potentials
array([  -868050.64349966, -39519040.47906478,               -inf])

>>> sum(s.chemical_potentials * s.X)
nan

# In contrast, entropy is finite
>>> s.partial_molar_entropies
array([   0.        ,  455.97960377, 2677.38624304])

# For other phases, chem potentials are finite even at zero mole fraction
>>> gas = ct.Solution('h2o2.yaml')
>>> gas.X
array([1., 0., 0., 0., 0., 0., 0., 0., 0., 0.])

>>> gas.chemical_potentials
array([-3.92042461e+07, -1.53944629e+09, -1.52217254e+09, -1.78457285e+09,
       -1.73880306e+09, -2.02150142e+09, -1.77920778e+09, -1.92926533e+09,
       -1.76944793e+09, -1.78048060e+09])

System information

• Cantera version: 2.6.0 or main at decda93

Additional context

This was discovered as part of #1299, which implements Cantera/enhancements#114.