Encapsulate nlohmann/json and magic_enum as implementation details

[PhilipFackler]

Description

These two third-party libraries don't need to be exposed to the user

Relates to #278
Needed for #253

Proposed changes

Introduce a simple function parseSystemModelData (which can take std::istream, std::filesystem::path, or std::string) to generate a SystemModelData<double, size_t> object from JSON input.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.

[pelesh]

lgtm