Add ConditionAccessor::get() to access 'current' condition

It is a simple wrapper around

get( owner->getConditionContect( Gaudi::Hive::currentContext() ) )

that simplifies adaptations of existing code to DD4hep compatible versions.

For example it avoids the need of boilerplate like Rec!2882 (diffs)

At a later stage the helper can be deprecated to easily identify the points that require clean up.

Requires Rec!2887 (merged) otherwise the build of Rec fails