F cpp to string

Summary

Implements #23 (closed) and fixes #24 (closed)

For #23 (closed)

The ProtoMessageWrapper has been refactored into an abstract base class for RepeatedPtrFieldWrapper and ScalarProtoMessageWrapper, which implement the ToString methods. Effectively this means that now Parent, Parents, Property, Properties, Message, Messages, DataType, Value, ScalarValue (and probably some more) classes have the ToString method.

For #24 (closed)

The problem was that the Value's collection_value member has not been overridden, when the Value itself has been overridden.

This has been fixed by initializing a fresh instance of Value and by resetting the collection_value member in the assignment operators.

This problem most likely affected the DataType as well (and should be fixed by now).

Focus

For #24 (closed)

1. Resets the collection_value: !25 (diffs)
2. Initialize new DataType !25 (diffs)
3. Initialize new Value !25 (diffs)

For #23 (closed)

Everything else is for #23 (closed).

Test Environment

• unit tests have been written.
• the cppinttest dev branch works for this as well

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• Reference related Issues
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  • Intent of new code
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• All automated tests pass
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doc/README_SETUP.md

@@ -34,6 +34,11 @@ have to add `build/lib/` (or, alternatively after installation,
 `CMAKE_INSTALL_PREFIX/lib`) to your `DYLD_LIBRARY_PATH` environmental
 variable.
 
+#### Problems and solutions ####
+
+- Make sure that your conan version supports your XCode version.  A typical symptom of version
+  mismatch is for example conan complaining about incompatible `compiler.version` settings.
+
 ### How to build on Windows
 
 We use [Visual Studio 2019](https://visualstudio.microsoft.com/de/vs/features/cplusplus/)