Parse documents into interlinked object structures

[lanthaler]

It should be possible to work directly with the data contained in a document. Some functionality that should be supported:

// load a document
$doc = Document::load('document.jsonld');

// get the default graph
$graph = $doc->getGraph();

// get all nodes in the document
$nodes = $graph->getNodes();

// retrieve a node by ID
$node = $graph->getNode('http://example.com/node1');

// get a property
$node->getProperty('http://example.com/vocab/name');

// add a new blank node to the document
$newNode = $graph->createNode();

// link the new blank node to the existing node
$node->addPropertyValue('http://example.com/vocab/link', $newNode);

// even reverse properties are supported; this returns $newNode
$node->getReverseProperty('http://example.com/vocab/link');

At a later point, it should also be possible to query for nodes or properties.

[lanthaler]

Merging graphs, currently doesn't work as blank nodes aren't relabeled properly. There's a bigger problem with the current implementation as nodes are generated at the graph level instead of the document level. This means that blank node graphs which are either shared between graphs or also used a graph names might get relabeled differently.