Link level buoyancy should not need top-level model scope

[srmainwaring]

PR #65 introduced the ability to enable hydrodynamics at link level using an <enable> element.

Currently the link must be fully scoped in the namespace of the enclosing model. This limits the reusability of models containing the hydrodynamics plugin as when they are including with a <name> element, the scoped name of the link changes and the hydrodynamics is not applied.

The proposed change is to not require the immediate enclosing model's name in the scoping rules.

Examples:

• model::float_link would be referred to as float_link
• model::nested_model::float_link would be referred to as nested_model::float_link

where we suppose that the plugin is defined in the scope of model.

Questions:

• Can the change be made so that is backwards compatible (or so the behaviour can be deprecated?)