Support Model JSON Serialization

[johannes-wolf]

Adds a toJson() to ModelPool and ModelNode + converting an object to a string will give a JSON representation.

Marks Fields::resolve as const.

[github-actions[bot]]

Test Results

 1 files  ±0   1 suites  ±0   3m 9s :stopwatch: -1s 34 tests ±0  34 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  35 runs  ±0  35 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit b0f3456f. ± Comparison against base commit fdaef25b.

This pull request removes 1 and adds 1 tests. Note that renamed tests count towards both.

``` Switch Model Fields Dict ‑ Switch Model Fields Dict ``` ``` Switch Model String Pool ‑ Switch Model String Pool ```

:recycle: This comment has been updated with latest results.

[sonarcloud[bot]]

Quality Gate passed

Issues
52 New issues
31 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions[bot]]

Package	Line Rate	Branch Rate	Health
include.simfil	61%	43%	➖
include.simfil.model	87%	61%	✔
src	73%	42%	➖
src.model	80%	47%	✔
Summary	74% (2058 / 2790)	43% (1660 / 3835)	➖

Minimum allowed line rate is 50%