Enable nullable reference types

[ygra]

Long list of commits, but perhaps that makes reviewing a bit easier, as the changes are somewhat scoped.

• Nullable reference types are enabled for the library and the example project
• All nullable warnings are errors in the library
• I've managed to have no remaining islands of #nullable disable and the build succeeds and the example still runs.

There have been a few problematic places, though, that have been annotated with comments, and sometimes pointed out in the commit message. Most of those have been silenced for now with a somewhat minimal set of ! operators.

See #230.

TODO (F):

• after merge, create an issue about
  • ScriptsAtom.BaseAtom being (supposedly) non-nullable
  • make StyledAtom's constructor parameter atom non-nullable, add asserts
  • make TypedAtom.Atom non-nullable, add asserts
  • UnderlinedAtom.Atom
  • VerticalCenteredAtom.Atom
  • OverUnderBox.ScriptBox
  • PredefinedFormulaParser shouldn't return null? to fix predefinedFormula!.RootAtom! (TexFormulaParser.cs:648)
  • Radical.BaseAtom
  • remove TODO / fix the issue with RowAtom ctor RowAtom(SourceSpan? source, IEnumerable<Atom?> elements)
  • fix an issue with RowAtom(SourceSpan? source, Atom? baseAtom) (see the comment)

[ygra]

OK, I do see build failures on my end as well, but seemingly only for net452:

Seems like the Nullable NuGet package doesn't seem to work as expected.

error CS0246: The type or namespace name 'NotNullWhenAttribute' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'NotNullWhen' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'NotNullWhenAttribute' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'NotNullWhen' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'NotNullIfNotNullAttribute' could not be found (are you missing a using directive or an assembly reference?)
error CS0246: The type or namespace name 'NotNullIfNotNull' could not be found (are you missing a using directive or an assembly reference?)
1>Done building project "WpfMath_xqner3n1_wpftmp.csproj" -- FAILED.

The build seems to work for netcoreapp3.0, which results in the compiled assembly. Subsequent builds in VS (Ctrl+Shift+B) then tell me "Build successful", as the build system apparently thinks all artifacts are there.

[ygra]

Apparently this is the problem: manuelroemer/Nullable#11

I guess I'll probably go with conditional compilation for the respective lines and we'll miss a bit nullability fidelity in net452.

[ForNeVeR]

Hell of a work you did here, thanks a lot! I've reviewed every single change, and left some comments.

I've decided to move some of the remaining work (see the TODOs I've added to the initial post) to a new issue though.

[ForNeVeR]

Thanks for your work!

[ygra]

Well, thanks for giving me an opportunity to try such a migration. Still planning to do so at work, albeit on a larger, more confusing code base where the only benefit is that I know it fairly well :-)

And I fear the amount of left over todos is probably normal for old code.