* Fix -Wsubobject-linkage warning
Having tk::spline header-only implementation included from
SmallAreaInfillFlowCompensator.hpp makes
SmallAreaInfillFlowCompensator::flowModel have separate (albeit the
same) implementation in each translation unit.
In order to fix this issue, SmallAreaInfillFlowCompensator::flowModel
converted to opaque 'pimpl'
* spline: remove anonymous namespace
Remove outer anonymous namespace from splice.h to make
forward declaration for tk::spline possible.
e6ed93f0c6