Add option to choose interpolation type.

* Choose either linear or constant for direct forcing interpolation.
* Do it via command line option -interpolationType.
* Add enum interpolationType.
* Pass this option to cavityRe100.py.
Direct forcing cases for constant interpolation.

* Reorganize folders.
Add 'virtual' to functions in derived classes.
Print info about the mass fluxes in the domain.

* Write the max, min and sum of divergences in all cells in the domain.
Reorganize folders for FadlunEtAl cases.

* Edit README (remove the 'last updated' line at the top.)
Matplotlib scripts to plot pressure and vorticity.

* Rename the script that uses gnuplot to plotVorticityGnuplot.py.
* Add function readPressureData to readData.py.
* Change names of command line arguments in plotVelocity.py.
* Other minor edits.
Switch to using a single tags array.

* Also single arrays for coeffs and uv.
* Perform interpolation only along one direction.
* The convergence rate is not the same as before - investigate this.
Fix bug: Correct spelling to "simulation".

* This was in file tagPoints.inl, where interpType is initialized.
* Future idea: Change parameterDB so that this throws an error?