Replaced make_cell_hierarchy.sh with a more flexible Python version.

Replaces the bash script that merged multi-rank cell_hierarchy*.csv files into one file and copied the .html file used for interactive viewing with a Python version. The new script has a number of new features compared to the bash version:

• Support for a custom output file name, so that output files for multiple steps can be generated in the same folder
• A new --serve command line argument that launches a simple socketserver.TCPServer on the localhost, so that the generated output file can be displayed directly without the need to copy it to a web server.

I wanted to assign this to @lhausammann, but I'm not quite sure he still gets these emails.