blockMesh: Add support for multi/sectional grading in a block

Consider a block describing a channel with two opposite walls.
Currently in order to grade the mesh towards the walls and have a
uniform region in the centre the channel would need to be spit into 3
blocks.  With the new multi/sectional grading this can be achieved in a
single block e.g.

blocks
(
    hex (0 1 2 3 4 5 6 7) (20 60 20)
    simpleGrading
    (
        1
        ((0.2 0.3 4) (0.6 0.4 1) (0.2 0.3 0.25))
        1
    )
);

In this example the block is divided uniformly in the x and z -directions
and split into three grading sections in the y-direction described by
three triples:  ((0.2 0.3 4) (0.6 0.4 1) (0.2 0.3 0.25)).  Each of the
grading sections is described by a triple consisting of the fraction of
the block, the fraction of the divisions and the grading ratio (size of
first division/size of last division).  Both the fraction of the block
and the fraction of the divisions are normalized automatically so they
can be specified scaled in anyway, e.g. as percentages:

blocks
(
    hex (0 1 2 3 4 5 6 7) (20 60 20)
    simpleGrading
    (
        1
        ((2 3 4) (6 4 1) (2 3 0.25))
        1
    )
);

and they need not sum to 1 or 100.

This is very new functionality and not well tested but backward
compatibility has been well tested so all existing blockMeshDicts should
parse correctly.