2D Coordinate Systems in RAVL

In 2D RAVL objects, such as arrays, matrices and images, the
indices are in the order: row, column (as in conventional matrix
notation). In pairs of reals (e.g. 2D vectors and points), the
components are in the order *x*, *y*. Normally these
two categories of objects do not interact. However, when they
do (e.g. if we want to construct a pixel position, an
`Index2dC` from a
`Point2dC`), the
RAVL convention is this:

- The
**row** index is associated with the *x* value
- The
**column** index is associated with the *y* value

The implication is that RAVL adopts a 2D coordinate system in which the
*x* coordinate points downwards, and the *y* coordinate to the right.
Not what you were expecting? It has the advantages that:

- the coordinate system is anti-clockwise, as is conventional in mathematics
- the first array coordinate is associated with the first vector component etc.