File lists and Boutiques-based tools

[MontrealSergiy]

Intro:
CBRAIN supports import of tools in Boutiques format. Regretfully, input parameters of FIle type in Boutiques tool descriptors, are not properly supported in CBRAIN, while, I think users and tool developers could benefit from full file list support.

An example of the file list input in Boutiques format:

        {
            "id": "list",
            "value-key": "[list]",
            "list": true,
            "command-line-flag": "--list",
            "description": "Some file list. Could be e.g. the list of files with MRI data to analyze.",
            "type": "File",
            "name": "files (with MRI data_"
        },

(see https://raw.githubusercontent.com/MontrealSergiy/cbrain-plugins-eeg/bugs/cbrain_task_descriptors/filelistbug.json for full descriptor)
CBRAIN seemingly imports boutiques descriptors with file lists without any warning or error message, configuring and executing tool's task is ridden with several issues.

Steps to reproduce issues:

1. Create boutiques with a file list input, download from above GitHub link, ( or copy one from Boutiques tests).
2. Import boutiques into CBRAIN;
3. Create a task and a) try to populate the input with more than one file b) run it (with one file list).

Observed: User can only supply one file per input, and, anyway, the command line will be malformed and (usually) fail

Expected: User should be able to provide multiple files and put them into file list, perhaps in the same way he can create integer or string lists

Note 1: The lists of numbers or strings are supported by CBRAIN, the problem is with file lists

Note 2: Older versions of boutiques had a few issues with file lists, which are fixed by now

[prioux]

Implemented in #832