Add support for converting BULK phase definitions from Chemkin input files

[speth]

Chemkin surface input files allow a bulk (solid) phase to be defined using the BULK keyword. Handling of this keyword is not currently implemented in ck2yaml. Such phase definitions are necessary for mechanisms that have net deposition / ablation reactions.

My interpretation of the Chemkin format is as follows:

• The section starts with the keyword BULK
• The BULK keyword may optionally be followed by a name for the bulk phase, surrounded by /
• This is followed by one or more species definitions. Each species definition consists of a name and, optionally, the density of the species in g/cm³ surrounded by /.
• The section may optionally end with the keyword END
• The section can continue across multiple lines.

Examples:

• BULK /Silicon/ Si(B)/ 2.33/ defines a phase named "Silicon" with a single species.
• BULK Si(B) defines a single species phase with no density information and no name.
• BULK Si(B)/2.33/ SiO2(B)/3.4/ defines an unnamed phase with two species

For single-species phases, I think these definitions could be interpreted as fixed-stoichiometry phases with constant-volume as the equation-of-state model for the species.

For multi-species phases, I think the best default would be the ideal-solution-VPSS phase model with constant-volume as the equation-of-state model for each species.

Support for writing these phase definitions out when required should also be added to yaml2ck.

Additional context

Issue noted on the Users' Group here, among others.