Unclear error message when the dataset and processing configs are swapped on the command line

[benkrikler]

The error message, at the moment (v0.12.0), when a user provides the dataset and processing yaml files the wrong way round is not clear at all. This is the current error and backtrace:

$ fast_carpenter --outdir output_test/ configs/explore_example_unbinned_dataframe.yaml samples/control_regions_skim_2017_Nano14Dec2018/WJetsToLNu_HT-200To400.yml 
Traceback (most recent call last):
  File "/afs/cern.ch/user/d/davignon/miniconda2/envs/test_carpenter2/bin/fast_carpenter", line 10, in <module>
    sys.exit(main())
  File "/afs/cern.ch/user/d/davignon/miniconda2/envs/test_carpenter2/lib/python2.7/site-packages/fast_carpenter/__main__.py", line 66, in main
    sequence = fast_flow.read_sequence_yaml(args.sequence_cfg, output_dir=args.outdir)
  File "/afs/cern.ch/user/d/davignon/miniconda2/envs/test_carpenter2/lib/python2.7/site-packages/fast_flow/v1/__init__.py", line 13, in read_sequence_yaml
    return read_sequence_dict(**cfg)
TypeError: read_sequence_dict() takes at least 1 argument (1 given)
(test_carpenter2) [davignon@lxplus606 analysis]$ fast_carpenter --outdir output_test configs/explore_example_unbinned_dataframe.yaml samples/control_regions_skim_2017_Nano14Dec2018/WJetsToLNu_HT-200To400.yml 
Traceback (most recent call last):
  File "/afs/cern.ch/user/d/davignon/miniconda2/envs/test_carpenter2/bin/fast_carpenter", line 10, in <module>
    sys.exit(main())
  File "/afs/cern.ch/user/d/davignon/miniconda2/envs/test_carpenter2/lib/python2.7/site-packages/fast_carpenter/__main__.py", line 66, in main
    sequence = fast_flow.read_sequence_yaml(args.sequence_cfg, output_dir=args.outdir)
  File "/afs/cern.ch/user/d/davignon/miniconda2/envs/test_carpenter2/lib/python2.7/site-packages/fast_flow/v1/__init__.py", line 13, in read_sequence_yaml
    return read_sequence_dict(**cfg)
TypeError: read_sequence_dict() takes at least 1 argument (1 given)

This was hit by @davignon. We need a clearer message to help users, or switch to non-positional arguments for the main command.

[benkrikler]

I think the best way to handle this will be to switch from positional command-line arguments to ones indicated with an option but are required. So the command-line would change from:

fast_carpenter datasets.yml steps.yml

to something like:

fast_carpenter -d datasets.yml -s steps.yml

It's also easier to generalise to having other configs, if we do this. However, since this will be a breaking change for current users, I might leave this off for version 1.0.