Add case for 2-D lid-driven cavity at Re=3000.

* Edit makefile.
* Change simulation parameters for Re=3200, 5000 cases.
* Write dt to simulationInfo files.
* Edit plotting scripts to read dt from the Info files.
* Edit cavity.py
  * Output the centerline velocities into a text file
  * Draw plots for cases when validation data is not available.
  * More error messages when a problem is encountered.