Changes proposed in this pull request

The main purpose of this PR is to modify the implementation of ExtensibleRate so that the core Cantera library (libcantera) does not need to depend on a specific Python installation at compilation / installation time. Achieving this required some fairly far-reaching changes, though I think they will all have long-term benefits.

• The PythonExtensionManager class and associated Cython functions have been moved out of libcantera into a separate shim, libcantera_python that is loaded on demand. This new library is the one that links to libpython and loads the Cantera Python module.
• Because a user C++ program and the Cantera Python module (loaded via libcantera_python) both depend on libcantera, libcantera needs to be loaded as a shared library to avoid having two copies of the library with separate state (which would interfere instantiating user defined types that are only instance with one copy of ReactionRateFactory, for example). Multiple versions of Python are supported (not at the same time!) by having multiple versions of libcantera_python, e.g. libcantera_python3_11.so and libcantera_python3_10.so.
• On Windows, linking C++ applications and the Cantera Python module to libcantera_shared requires "exporting" the complete C++ interface. Due the the sheer number of methods that need to be exported, rather than the standard approach investigated in Export C++ library symbols for core methods #1298, this PR uses the static library to generate a list of symbols to be exported from the DLL.
• The functions to load the libcantera_python library on-demand are OS-specific. Rather than use these low-level functions, the Boost.DLL library is used for its higher-level, cross-platform capabilities. While Boost.DLL is header-only, by default it depends on Boost.Filesystem, which has a compiled component. However, since C++17 introduced std::filesystem (derived from Boost.Filesystem), Boost.DLL provides an option to use std::filesystem. Therefore, in order to avoid introducing a dependence on the compiled components of Boost, this PR bumps Cantera's compiler requirement to the C++17 standard.

There are also a few more minor changes that ended up coming along for the ride:

• Add using directives within the Cantera namespace for some of the most-used standard library types: string, vector, map, function, pair, unique_ptr. I figure map<string, string> is more readable than std::map<std::string, std::string>.
• Bump vendored versions of fmt and Eigen.
• Move some function definitions out of the header files, especially in the case of the Factory classes.

If applicable, fill in the issue number this pull request is fixing

• Continues the work on Enable development of models written in high-level languages (e.g. Python) enhancements#79
• Provides a path to resolving Update to handle introduction of Python extension support conda-recipes#40
• Resolves Export C++ symbols in dynamic Windows libraries enhancements#140
• The underlying functionality here should enable implementation of Enable calling of pre-compiled kinetics in CKWYP format enhancements#126

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review