GetSegment wrapper with Parametric and EndValues instances

For extracting individual segments from trails by parameter.  This
commit only includes instances for lines, need to add loops and
trails.
D.Trail: Parametric and EndValues instances for GetSegments on loops and tails
D.Trail: document semantics of Parametric GetSegment at segment junction
D.Trail: add reparameterization iso to result of Parametric GetSegment
new module D.Tangent: computing tangent + normal vectors
Merge pull request #113 from diagrams/tangent

API for computing tangent and normal vectors to segments and trails