Literal datatypes are missing in the Tabular Annotatation-generated mapping

[vcutrona]

Datatypes specified while annotating columns from the Tabular Annotation view are discarded in the automatically generated RDF mapping.

How to reproduce:

1. Create a new transformation and upload a file
2. Switch to the Tabular Enrichment tab
3. Annotate a column (A) as a URI column
4. Annotate a column (B) as a literal, specify its datatype (e.g., xsd:string), and set A as its source column.
5. Click the "Update RDF mappings" button
6. Switch to the RDF Mapping view and select the Tabular Annotation mappings tab in the left panel
7. Edit the Literal node created for column B

RESULT: the datatype is missing and the toggle "Assign a data type" is off.

This error prevents the transformation to properly work (see #93).