Make various subclasses of Surface::Sampler private

They are all used only by their containing classes, and so don't need
to be public.
Move private nested classes of Cylinder into cylinder.cc
Move private nested classes of Ellipse into ellipse.cc
Move private nested classes of Sphere into sphere.cc
Move private nested classes of Sphere2 into sphere2.cc
Move private nested classes of Tripar into tripar.cc
mesh.cc whitespace tweaks
Slight method rearrangement in mesh.cc

... just to consolidate things a bit.
Move definition of Mesh::Triangle::IsecInfo into mesh.cc