Plotting nodal data

[ammarhakim]

At present, pgkyl is a cell-based program. That is, the assumption is that the data is an expansion in each cell (so FV and DG). This means that the number of cells and total size of arrays is the same. However, we should also support nodal data. In this case the number of cells in the grid will be fewer than the total number of array elements (as there is 1 extra node in each direction than cells).

Perhaps this should be done as part of refactoring of the readers in the Issue #39. However, this is required as we go to more complex simulations in which some data may be nodal (for example, nodal temperatures and cell-centered heat-fluxes). At present it seems impossible to plot such fields, I think.