This is draft of what need to be done in order to support nullable reference types.
This commit actually should be split into several smaller commit:

1. Annotate folder after folder, starting with signatures
2. Annotate top-level files
3. Annotate demo-applications (right now only core library is annotated)

Amount of warnings should be reduced to 0. Right now ~200 of them.

Some classes should be reworked, so they wouldn't get into invalid state, for example Image. It should have Bitmap set, but it's not enforced by constructor

Also there's couple of fixes that should be in different PRs

This PR is a great example of how to nullables (and some switch expressions) in the core ScottPlot code base. Thanks @jl0pd! Unfortunately there remains ambiguity as to whether or not .NET Standard libraries (which use C# 7.3 by default) should be compiled using features of higher language versions, as discussed in #691. I'll close this PR for now, and we may open it up again depending on how that conversation progresses.

This PR is very insightful, and the steps described by #692 (comment) are excellent advice. Thanks @jl0pd for all the work you put into this! If/when the language version issue is resolved (or when we deprecate .NET Framework support) and the language version can be increased with confidence, this will be a great starting point to add nullable support throughout the library 👍