overhauling elements / shapes /symbols api

[dfreeman06]

The usability can be improved with a clearer api surface area.

• promote the contrib.elements to ipyelk.elements
• adding metadata to BaseElement
• fix hover exception for some elk ids not in the transformer registry
• standardizing transformer registry items to Marks
• convert Compound class to MarkFactory
• rework Symbol and Shape schema and api.
• rename ConnectorDef aka ElementSymbol offset fields

[Santiago Balestrini-Robinson]

Changing these ValueErrors to ElkRegistryErrors would be a nice cheap quality of life improvement:

• https://github.com/jupyrdf/ipyelk/blob/master/py_src/ipyelk/app.py#L90
• https://github.com/jupyrdf/ipyelk/blob/master/py_src/ipyelk/app.py#L113