SinkTree Separation and Cleanup

[sbondorf]

• The Backlog bound moved from the generic code to the sinktree package.
• Issue #51 is solved by this -- the AlgDNC backend defines which de-/convolution to be used, not calling a sinktree-specific method.
• Better enum names: _DIRECT suffix tells that the algDnc backend configuration will be ignored as the code directly works with the curve segments instead of using the discodnc code for opertions.
• TbRl to Affine renaming.