Add species alias

[ischoegl]

This PR adds utility functions that facilitate selection of species and isomers. Functions are defined in the C++ layer and broken out using the Cython interface.

Changes proposed in this pull request:

• add_species_alias allows for alternate species names/shorthands
• find_isomers returns a list of isomers for a given composition

Usage:

In [1]: import cantera as ct

In [2]: gas = ct.Solution('nDodecane_Reitz.yaml')

In [3]: gas.find_isomers({'C':4, 'H':9, 'O':2})
Out[3]: ['c4h8ooh', 'c4h9o2']

In [4]: gas.find_isomers({'C':7, 'H':16})
Out[4]: []

In [5]: gas.find_isomers('C:1, H:4')
Out[5]: ['ch4']

In [6]: gas.add_species_alias('ch4', 'Methane')

In [7]: gas.species_index('Methane') == gas.species_index('ch4')
Out[7]: True

In [8]: gas.X = 'Methane:1.'

In [9]: gas.X[gas.species_index('ch4')]
Out[9]: 1.0

PS: the implementation of findIsomers is analogous to #635; this PR explicitly does not address the overloading of speciesIndex (which could be simplified based on this PR).

[speth]

The addition of findIsomers makes sense to me.

I don't see much of a direct use case for addSpeciesAlias, but I can imagine ways of using it if you had some extra data in your input file, e.g. the SMILES corresponding to each species, so adding some underlying infrastructure for this kind of feature seems fine.

[ischoegl]

I don't see much of a direct use case for addSpeciesAlias [...]

Sometimes I find large molecules with non-intuitive species names, and keep having to remind myself what they actually are. Those names are also not consistent among groups; as m_speciesIndices is already a map, it's pretty simple to add this feature.

[codecov]

Codecov Report

Merging #714 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #714      +/-   ##
==========================================
+ Coverage   70.85%   70.86%   +0.01%     
==========================================
  Files         374      374              
  Lines       43668    43683      +15     
==========================================
+ Hits        30939    30954      +15     
  Misses      12729    12729

Impacted Files	Coverage Δ
include/cantera/thermo/Phase.h	100% <ø> (ø)	⬆️
src/thermo/Phase.cpp	82.24% <100%> (+0.58%)	⬆️
src/transport/GasTransport.cpp	90.58% <0%> (ø)	⬆️

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