fix missing presentation def in json schema

[wolf4ood]

WHAT

fix missing presentation def in json schema

Closes #97

How was the issue fixed?

Briefly state why the change was necessary.

More context

List other areas that have changed but are not necessarily linked to the main feature. This could be naming changes,
bugs that were encountered and were fixed inline, etc.

[wolf4ood]

/cc @arnoweiss

[arnoweiss]

Does it make sense to define the presentation property without defining the presentation definition object's structure? This could be tricky because the object is governed outside of DCP. I'd put into question if we need it at all.

[wolf4ood]

Unless we want to profile it, probably it worth investigating a schema reference to the presentation definition.

[arnoweiss]

Ok, I'll open another issue for this. Thanks for the initial addition here.