WIP: Pydarshan file_based sorting

[Yanlilyu]

Description:

• This branch adds a file_based routine to help user find the most I/O intensive files. The file is file_stats.py in PyDarshan CLI tools.
• This branch also adds a test_file_stats.py to PyDarshan tests to test file_stats.py.
• It combines the data from multiple log files to a DataFrame, groups the data by “id”, sorts data by the column name the user inputs in a descending order, and then filters the data with the first n (number_of_rows from user input) records. It returns a DataFrame with n most I/O intensive files.
• User input includes log_path, module, order_by_colname, number_of_rows. The command line arguments are name arguments.
• log_path should be a list of files or a shell glob.
• The default values for module, order_by_colname, number_of_rows are “POSIX”, “POSIX_BYTES_READ”, 10, respectively. If users don’t input these values, the tool will use default values.
• The tool checks if the module is in the list of modules. If not, it prints an error out and exits immediately.
• order_by_colname should be “{mod}_BYTES_READ” or “{mod}_BYTES_WRITTEN”.
• The tool also checks if the order_by_colname the user inputs is consistent with the module. For example, if the module and order_by_colname are POSIX and STDIO_ BYTES_READ, there will be an error “Column name should be ‘{mod}_BYTES_READ’ or ‘{mod}_BYTES_WRITTEN’“.
• Example usage:
  $ python -m darshan file_stats darshan_logs/nonmpi_workflow/worker*.darshan -m STDIO -o STDIO_BYTES_READ -n 5
  $ python -m darshan file_stats darshan_logs/nonmpi_workflow/worker*.darshan
  $ python -m darshan file_stats darshan_logs/nonmpi_workflow/worker_1.darshan darshan_logs/nonmpi_workflow/worker_3.darshan -m STDIO -o STDIO_BYTES_READ -n 5