[SIC\]DIFF Var1|File1 Var2|File2
List the differences between two entities. Each entity can be either a
Sic variable name or a GDF file name (images, cubes, UV tables,...).
Comparison is made on their headers (if both variables provide a header)
and/or their data (if both variables provide a data array).
The result of the last comparison (true/false) is saved in the logical
variable SIC%DIFF.