Allow fields to require a value be provided on deserialiation.

[Crell]

Description

Adds a new boolean to the Field attribute to require the property. Also a new boolean field to the class to require any property that doesn't specify otherwise.

Given how easy it is to set on a class level, I am going to leave all the defaults at false. It's messing with a lot of other tests otherwise.

I've also decided that if a field is nullable, that means nothing. If you want a default value of null in case of a missing value... set a default value. That support already exists.

Motivation and context

Fixes #7 Fixes #14

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• [X] New feature (non-breaking change which adds functionality)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

Please, please, please, don't send your pull request until all of the boxes are ticked. Once your pull request is created, it will trigger a build on our continuous integration server to make sure your tests and code style pass.

• [X] I have read the CONTRIBUTING document.
• [X] My pull request addresses exactly one patch/feature.
• [X] I have created a branch for this patch/feature.
• [X] Each individual commit in the pull request is meaningful.
• [X] I have added tests to cover my changes.
• [X] If my change requires a change to the documentation, I have updated it accordingly.

[Crell]

@repli2dev This work for your use case?

[repli2dev]

@Crell it seems it should, yet with a bit more writing, probably acceptable.