Added checking of IOD-specific constraints.

Enhanced new insertSubTree() method by also checking whether the content
relationship constraints of the underlying SR IOD are followed, which is
needed for the derived class DSRDocumentTree. Also added further tests.
Fixed two memory leaks in recently added code.

Fixed a memory leak in DSRDocumentSubTree::insertSubTree() and another one in
the destructor of class DSRContentItem. These leaks were found with valgrind.
Clarified use of template identification.

Made it clearer that the template identification using the Content Template
Sequence (0040,A504) is only applicable to CONTAINER content items. At the
time when the development of the "dcmsr" module started, this restriction was
not present in the DICOM standard (see DICOM PS 3.3-2000 for details).
Specific error if no presentation context proposed.

Return specific error if Association requestor does not propose any
presentation context (must propose one or more).
Reverted false retirement status of some SOP Classes.

Three composite instance retrieve SOP Classes have accidentially
been added using the "RETIRED_" prefix in their respective constant
names. Thanks to Michael Spacek <tupinek@gmail.com> for the report
and suggested patch.
Fix last commit by including implementation file.