Fix seq_id/id bug in reader

[ppebay]

As reported by @lifflander:

# Model configuration:
  is_fwmp: False
  alpha: 1.0
  beta: 0.0
  rank_memory_bound: 4000000000.0
  file_stem: /Users/jliffla/codes/virtual-transport/gemma-test-problems/vt.3x.3n.24rpn.2tpr.3sub.2000rows.2000cols.measjson.nolb/data
# Number of detected ranks: 72
Traceback (most recent call last):
  File "/Users/jliffla/codes/virtual-transport/ccm-milp/src/ccm_milp_run.py", line 217, in <module>
    main()
  File "/Users/jliffla/codes/virtual-transport/ccm-milp/src/ccm_milp_run.py", line 186, in main
    data = Generator.parse_json(files, rpn, rank_mem_bnd, node_mem_bnd, verbose)
  File "/Users/jliffla/codes/virtual-transport/ccm-milp/src/ccm_milp/generator.py", line 630, in parse_json
    data.parse_json(data_files, verbose)
  File "/Users/jliffla/codes/virtual-transport/ccm-milp/src/ccm_milp/data.py", line 128, in parse_json
    com["from"]["seq_id"],
KeyError: 'seq_id'