Deprecate magic numbers in zeroD CLIB interface

[ischoegl]

Cantera version

All / current development version (2.5.0a2)

Operating System

All

Python/MATLAB version

N/A

Expected Behavior

The use of magic numbers as type constants should be deprecated throughout Cantera in favor of string types. I.e. type queries should have a signature virtual std::string type() const, like in class ThermoPhase.

Actual Behavior

All current objects in zeroD still use magic numbers with a signature int type() const. While a resolution of this issue in the cython interface is straight-forward, the current structure of the CLIB interface depends on integer-based object creation, i.e. internal integer m_type class variables are still essential.

Steps to reproduce

N/A

[ischoegl]

This Issue is based on comments by @speth (made during discussions for PR #632). A corresponding fix is proposed in PR #660.