Implementation of SDX' own concept of document.

SDX being a search engine for XML documents, the concept of document is central in its model. The class hierarchy give some hints on basic properties of documents in SDX :

- Parsable documents are documents that can be parsed, directly or not, by an XML parser. Such documents must implement the {@link fr.gouv.culture.sdx.document.ParsableDocument ParsableDocument} interface.
- Indexable documents are documents that can be indexed by SDX. Such documents must implement the {@link fr.gouv.culture.sdx.document.IndexableDocument IndexableDocument} interface.

The class {@link fr.gouv.culture.sdx.document.AbstractDocument AbstractDocument} is a conveninent class for implementing new types of documents.

In the near future, HTML documents will be supported, both as parsable dans indexable documents. Other document types such as PDF could be made indexable in the future.