Define the PATCH requests more detailed

[alexgordtop]

What?

It would be nice to have more details about PATCH requests specified.
e.g.

• which object attributes are mandatory?
• Is there a difference between "value=null" and "value" attribute not contained in request?
• In case of Entity, SubmodelElementCollection, ... -> What happens, if only some or no child elements are submitted?

How?

@FrankSchnicke @mjacoby Did I forget something?

• I have signed the required Developer Certificate of Origin (DCO) already.

[FrankSchnicke]

The PATCH on /submodel-elements is missing. It is defined for the submodel itself as well as for a specific SubmodelElement but not for the overall list.

[mjacoby]

see #236