Adding More Cases and Auxilliary Files

[smullangi3]

Adding:

• Revised velocity field code and also README to explain how to use it
• Carotid artery and Web simulations, as well as the wall geometries and associated README entries in the sample_files
• Parallelize collision detection to speed up random cell placement when using large # of processors -- distributes collision-detection workload using a grid-stride pattern and uses MPI_AllReduce to determine if a cell's placement is successful or not.