Documentation for importing packed files

Hello,

Been playing around with how to import a packed cwl json file as a CWL parser object.

Here are my steps so far

## Setup
```python
# Imports
from pathlib import Path
import json
import sys

# Set path
cwl_file_path = Path("/path/to/cwl.packed.json")

# Load file as dict
# Read in the cwl file from a json
with open(cwl_file_path, "r") as cwl_h:
cwl_file_dict = json.load(cwl_h)

# Conditional import based on cwl version
if 'cwlVersion' not in list(cwl_file_dict.keys()):
print("Error - could not get the cwlVersion")
sys.exit(1)
# Import parser based on CWL Version
if cwl_file_dict['cwlVersion'] == 'v1.0':
from cwl_utils import parser_v1_0 as parser
elif cwl_file_dict['cwlVersion'] == 'v1.1':
from cwl_utils import parser_v1_1 as parser
elif cwl_file_dict['cwlVersion'] == 'v1.2':
from cwl_utils import parser_v1_2 as parser
else:
print("Version error. Did not recognise {} as a CWL version".format(yaml_obj["CWLVersion"]))
sys.exit(1)
```

## First attempt:
> Use the load document feature
```python
parser.load_document(cwl_file_dict, cwl_file_path.absolute().as_uri())

SchemaSaladException: Cannot load $import without fileuri
```

## Second attempt
> Convert to string then load
```python
parser.load_document_by_string(json.dumps(cwl_file_dict), cwl_file_path.absolute().as_uri())

ValidationException: - tried _RecordLoader but
Expected a dict
- tried _RecordLoader but
Expected a dict
...
```

## Third attempt
> Convert to yaml then load
```python
# We need to import the ruamel yaml class
from ruamel import yaml
# Dump our dict to a yaml string
cwl_yaml_dump = yaml.round_trip_dump(cwl_file_dict, Dumper=yaml.RoundTripDumper)
# Load yaml
cwl_yaml_load = yaml.round_trip_load(cwl_yaml_dump, preserve_quotes=True)
# Import
parser.load_document_by_yaml(cwl_yaml_load, cwl_file_path.absolute().as_uri())

ValidationException: - tried _RecordLoader but
Expected a dict
- tried _RecordLoader but
Expected a dict
...
```

## Fourth attempt
> Convert just graph to yaml then load
```python
# We need to import the ruamel yaml class
from ruamel import yaml
# Dump our dict to a yaml string
cwl_yaml_dump = yaml.round_trip_dump(cwl_file_dict['$graph'], Dumper=yaml.RoundTripDumper)
# Load yaml
cwl_yaml_load = yaml.round_trip_load(cwl_yaml_dump, preserve_quotes=True)
# Import
parser.load_document_by_yaml(cwl_yaml_load, cwl_file_path.absolute().as_uri())

ValidationException: - tried _RecordLoader but
Expected a dict
- tried _RecordLoader but
Expected a dict
...
```

Is this due to my workflow being a little bit too complicated for the parser and using record schemas?