Serializer set namespace prefix

[pduchesne]

This PR adds the possibility to (forcefully) set namespaces prefixes on a Serializer instance, as opposed to the current suggestNamespaces and suggestPrefix methods that propose prefixes but cannot override defaults prefixes.

What it does is

• fix IndexedFormula.setPrefixForURI that did allow multiple namespaces to be mapped on the same prefix
• add setNamespaces and setPrefix methods on Serializer that can override default prefixes
• add a options.namespaces to the serialize function, that defines namespaces that should be forcefully set
• override the IndexedFormula.serialize method so that the current graph namespaces are kept as mandatory prefixes

As a result ,

const graph: IndexedFormula = ...;

graph.serialize(base, contentType, provenance);
// is equivalent to
serialize(provenance, graph, base, contentType, undefined, { namespaces: graph.namespaces });

and keeps the graph namespace prefixes, while

serialize(provenance, graph, base, contentType, undefined);

behaves as before, i.e. giving priority to default prefixes