Core API: add getPlayer() to Association (#667).

The `de.deepamehta.core.Association` interface provides generic player accessors:
{{{
DeepaMehtaObject getPlayer1()
DeepaMehtaObject getPlayer2()
}}}

The `de.deepamehta.core.Role` interface provides a new method:
{{{
DeepaMehtaObject getPlayer()
}}}

The `de.deepamehta.core.AssociationRole` interface provides a method to get the player object:
{{{
Association getAssociation()
}}}

BREAKING CHANGE

1 method is removed from the `de.deepamehta.core.Role` interface:
{{{
Association getAssociation()
}}}
This method was used to get the association the role is involved to.
There is no replacement for that method.

See #667.