Indicate the root using `--root` flag, not `rdfs:comment`

[bartkl]

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Is your feature request related to a problem? Please describe.
Currently, indicating the root is part of the SHACL model. That's not nice, because it's Avro specific (or at least specific to tree-shaped schemas). Not to mention the way we're modeling it using rdfs:comment and a designated value RootObject, which is really not very nice.

Describe the solution you'd like
Using a CLI flag to indicate the root instead of the `rdfs:comment "RootObject".

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Task

Description
Remove the hacky rdfs:comment "RootObject" semantics and replace it with a command-line flag --root to indicate the root.

Usage would look like this:

metamorph avro --root :BShape input/SHACL.ttl

Acceptance criteria

• avro command supports a --root flag on the CLI
• rdfs:comment "RootObject" semantics are removed