File extension

[Zigur]

Tackles issue #378

Behaviour:

1. Given this "data file" ProductNode with an "extension" property:

[
          "raw spectral data file",
          {
            "node_type": "data file",
            "is_input_to_next_protocols": {
              "value": true
            },
            "extension": {
              "options": ["mzML", "mzXML", "JCAMP-DX"],
              "value": "mzML",
              "newValues": true
            }
          }
]

All the file nodes will have a .mzML extension, taken from "extension.value"

2. If the "extension" property is missing or no value is supplied, a default .raw extension is appended to the file names.

This can be tested on the notebooks, e.g. here

[coveralls]

Coverage increased (+0.02%) to 71.139% when pulling baf177f on file-extension into 75d864a on develop.