add notebook for analyzing matrix sparsity strucuture

[multitalentloes]

This PR adds a notebook for reading in matrix market files.
The notebook processes all matrix market files in a folder to easily collect statistics across a full simulation.
The notebook then analyzes sparsity structure and collects the following statistics:

• Median and mean number of non-zero blocks per row
• Min and max number of non-zero blocks per row
• Sparsity structure of all blocks
• Checks if the matrix is symmetric

The PR will stay in draft while all the functionality is added, and I am open to suggestions for more ways to characterize the matrices of the linear solver.