Allow target fields for wildcard field mappings

Currently FieldMappings the do use a wildcard are not allowed to be mapped to an target field.

The main reason for this is that things like the following example

    dc:* > dct:*

that would copy all fields from the dc elements namespace to the dc terms namespace are not implemented.

However copying multiple fields to a single one are already supported but not accepted by the FieldMappings constructor. A possible usage scenario is to collect all references or literal values to create an disambiguation context. See the following listing for an example.

* | d=entityhub:text > dis:textContext
* | d=entityhub:reference > dis:refContext

The change is really trivial as one needs only comment the check in the constructor of FieldMapping that explicitly disallows those things.