Add support for multi-valued attributes through list types

[jpalmeida]

Currently, attributes can only take one value (a single category, a text, a number).

Support for multi-valued attributes could be obtained by supporting list types (similar to Gephi) (e.g., category list, text list, number list).

[paulgirard]

We introduced one multi-valued attributes in the v1.0 : keywords
You can cast one attribute into the keywords type in the data table by precising the value separator:

But we don't support multiple values for other types yet.