Makefile: revert back to using wildcards for sources
TNode: new ABC for TreeNode-like classes

Currently a mix of CRTP and virtual functions
Source: Derive from & implement TNode "interface"
Block::findSources use Source's TNode::collectIf()
TestSource: use new collectIf() and childCount()
Source: remove old findSources() and findSourcesRec()

This has now been replaced by TNode::collectIf
TNode: add documentation for collectIf
TNode: Implement simple predCollectTrue (returning true)
Source: use predCollectAll in sources()
const string & in Block
remove unused sources_ids method
EntityWithSources: use ReferenceList in sourceCount()
first implementation of DataTag
SimpleTag: use RepresentationList (fix #26)
Merge branch 'master' of https://github.com/G-Node/pandora
TNode: move documentation to the right place
Section: derive from & impl. TNode; remove findSections()

Use collectIf() in File and TestSection
Section: remove sectionCount obsoleted by childCount
Section: remove getSection obsoleted by getChild
CMake: add Doxygen support to enable make doc