Upscale size-independent properties

Some properties are independent of blocksize (such as the volumetric
source term) in the sense that they should not be averaged, but simply
summed to find the upscaled version.

This method encapsulate the index retrieval and summation, and keeps
us from having yet another layer of indirection. An outer loop is
still needed in order to keep working on each column, as the other
methods in the interface.
Expose a way to get the upscaled source term

Upscaled source terms are calculated from the corresponding information
for the coarse grid, and a method made available to get these data back
out again.

This method make a copy of the data, so if they change, then the
upscaled values are not updated.
Use upscaled source term

The previous versions sends an array of values, which must be larger
so the accesses are always present, but they are not valid to use as
source terms for the upscaled grid due to index differences.

(Unless one has no source terms and every item is zero, but that's
just a handy coincidence).
Merge pull request #25 from rolk/025_src

Upscale source terms