add a json parser for the grid dynamics case format

[superwhiskers]

this pull request adds a parser for the grid dynamics case format

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.

[superwhiskers]

i've written the dummy test and it appears to parse. i still need to poke the parsed data a little closer but it appears to work correctly

[superwhiskers]

supplanted by #159