Add BeApproximately version for both subject and expected nullable

[krajek]

Closes #922.

For now, I have tackled only the case of double?.
Please review. Upon approval, I will copy the logic and tests for float? and decimal?.

[krajek]

PR is ready.
Let me know what you think about NotBeApproximately(nullable, nullable[...]).
For me it makes sense, but it was not discussed not requested in the #922.
I will create another PR if we agree to add it.

[jnyrup]

@krajek to quote an incredible active contributor (hint, it's you!)

Good catch, thanks.

I didn't think about NotBeApproximately, but it makes good sense to add that as well the align the API overloads with BeApproximately.

[dennisdoomen]

Yeah, I wonder where @krajek is getting the time from.... ;-)