Fix/cpp gen storing yaml

[SGSSGene]

This PR addresses multiple concerns at once:

1. It adds special handling of CWL Expression type
2. It now handles simplification of types by respecting the typeDSL option
3. It also handles mapPredicate when doing map ⇔ list conversions. (and not only mapSubject)
4. added a DocumentRootType type and load_document and store_document functions.
5. Added a store_config object, which allows to control simplification of types, list to map conversion and partial YAML-tag output (all very helpful when debugging).
6. It wraps the generated code into a cpp_gen namespace to avoid symbol spilling into other parts of c++ programs.

A basic program that can load and save(print) CWL tool description files (or any DocumentRootType type):

#include "cwl_v1_2.h"

#include <iostream>

namespace cwl = w3id_org::cwl;
int main(int argc, char** argv) {
    if (argc != 2) return 1;

    auto tool = cwl::load_document(argv[1]);
    cwl::store_document(tool, std::cout);
    return 0;
}

[codecov]

Codecov Report

Attention: Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.29%. Comparing base (3dc90de) to head (5ef98ca).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
schema_salad/cpp_codegen.py	98.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #900      +/-   ##
==========================================
+ Coverage   83.13%   83.29%   +0.15%     
==========================================
  Files          22       22              
  Lines        4899     4944      +45     
  Branches     1177     1182       +5     
==========================================
+ Hits         4073     4118      +45     
  Misses        548      548              
  Partials      278      278              

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