Print eta values for direct forcing method.

* Analyze eta values in analyzeTags.py.
* Edit related files.
* Change output file type in cavityRe100.py to pdf.
Add convergence cases for 25x25 and 30x30 grids.
Write solver for Diffusion with immersed boundary.

* cavityRe100.py: Print convergence rates for a general number of grids.
* Calculate the average eta for 2-D direct forcing.
* Fix direct forcing case for cylinder with linear interpolation.
* Add cases for DiffusionSolver convergence test.