col:name and col:titles are not required,

[pvgenuchten]

especially in virtual columns the col:name and col:titles property should not be required

this PR sets them as None instead of throwing error

it enables col definitions such as:

{"columns": [
         {
           "virtual": true,
           "propertyUrl": "rdf:type",
           "valueUrl": "qudt:QuantityValue",
           "aboutUrl": "https://example.com/example3/{SAMPLE}/K/QV"
         },
         {
           "virtual": true,
           "propertyUrl": "qudt:unit",
           "aboutUrl": "https://example.com/example3/{SAMPLE}/K/QV",
           "valueUrl": "http://qudt.org/vocab/unit/MicroMOL-PER-KiloGM"
         }
]}