Enable all KNIME Cell Types as long as they are compatible to StringCell

Many KNIME cell types are compatible to StringCell.TYPE (e.g. all molecular cells (SDF, SMILES,...)). The current version of the python scripting extensions however insists that a cell type is >equal< to StringCell.TYPE.
Therefore I changed the check of the cell type to include a case where the cell type is compatible to the StringCell value class.)
I didn't want to break the code in [Attribute.java] (https://github.com/knime-mpicbg/knime-scripting/blob/development_2.9/knutils/src/de/mpicbg/tds/knime/knutils/Attribute.java), therefore I'm using getRawValue() instead of getNominalAttribute() to retrieve the cell's value.