Removed DLL specifier from template classes.

Removed DLL import/export specifier from template classes since this does not
seem to work with VisualStudio (when compiling with shared libraries enabled).
Slightly enhanced layout of Doxygen documentation.
Added public method to change SR document type.

Added public method that allows for changing the type of an SR document (IOD)
without deleting the currently stored document tree, if this is possible.
Fixed wrong term in Doxygen documentation.

Instead of "content relationship constraints" the DICOM standard uses the
term "relationship content constraints".
Added removeSubTree() method to subtree class.

Added removeSubTree() method to the document subtree class. This allows for
removing (and deleting) a particular subtree from an SR document tree/subtree.