improvements to the component signal object and json parser infrastructure

[superwhiskers]

this pull request just improves the documentation of the component signal object and removes direct usage of strings in exceptions where possible

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.