Map fine grid elements to their columns

This map is computed in the construction of the top surface, but is
not easily accessible later because it requires knowledge of the
accumulated number of inactive blocks before each location. We may as
well store the direct mapping instead, for speedier access.
Added warning about vertical faults
Make objects that contain alias for cube sizes

It is easier to address the top face with UP, rather than 5.
Provide example of col_cells usage
Define the individual block height member
Define reference heights for each column
Define total height of each column
Helper routine to find a specific face of a block
Helper routine to find the elevation of a face
Helper routine to find the height of a block
Calculate discrete heights for each block in column

This will be used to depth-average and to vertically integrate the
column later.
Note maximum storage required for columns

We then don't have to reallocate for each column, but can allocate a
buffer that we know is large enough to hold one column.
Define relative elevation from top of column

These sizes are precalculated and stored together with the rest of the
grid to avoid looping in calculating heights.
Calculate relative elevation for each grid block

This is needed when we shall calculate the hydrostatic pressure in the
upscaled capillary pressure.
Merge pull request #18 from rolk/018_heights

Compute heights for each block in each column