ENH: vector mag(), magSqr() methods - complementary to dist(), distSqr()

ENH: use direct access to pointHit as point(), use dist(), distSqr()

- if the pointHit has already been checked for hit(), can/should
  simply use point() noexcept access subsequently to avoid redundant
  checks. Using vector distSqr() methods provides a minor optimization
  (no itermediate temporary), but can also make for clearer code.

ENH: copy construct pointIndexHit with different index

- symmetric with constructing from a pointHit with an index

STYLE: prefer pointHit point() instead of rawPoint()

src/functionObjects/field/wallBoundedStreamLine/wallBoundedStreamLine.C

@@ -88,7 +88,7 @@ Foam::functionObjects::wallBoundedStreamLine::findNearestTet
                 const scalar d2 = nearInfo.distance();
                 if (d2 < minDistSqr)
                 {
-                    nearestPt = nearInfo.rawPoint();
+                    nearestPt = nearInfo.point();
                     minDistSqr = d2;
                     minFacei = facei;
                     minTetPti = i-1;