add additional notebooks to demonstrate pyvcell potential

[jcschaff]

The notebooks are now:

1. fvsolver_demo.ipynb: run finite volume pde solver from input files to generate outputs
2. data_demo.ipynb: read raw pde solver outputs and display with different plots
3. combined_fvsolver_data.ipynb: run PDE solver, then read and display raw results.
4. n5_download_demo.ipynb: download exported PDE data from remote N5 datastore using tensorstore
5. vcell_publications.ipynb: publication data using vcell api