Cleaned Gnuplot class
Defined ExportVariable as forward declaration.

Working on reducing deps on ExportVariable.
Cleaned and documented the OCP class
Moved diag(.) of ExportVariable to it's own class definition file.

-> Removed deps on ExportVariable.
Cleaned and extended the ExportVariable class

... + some minor cleaning in the ExportArith..Statement class.

Added getElement method to access a single element of an ex. variable.