^^xmls types for special values

[ghost]

Situation:

• a data column contains (decimal) numbers, but interspersed with occasional entries like "undefined", "null", "n/a" or similar.
• Only the object data type "string" can be selected for this column.

Then, in the transformation result, all objects get the type ^^xlms:string,
regardless of they really are numbers or other values.
This should probably be corrected as follows:

• It should be possible to select "decimal" as datatype
• numbers should get "decimal", words should get "string" as datatype.

As a possible extension, it should be possible to suppress the generation of a result triple in the case of a non-conforming field entry , at the user's discretion.