Add handling of ItemValue and SubItemValuePerItem into the language

- SubItemValuePerItem is associated to the pugs abstract type sub_item_value
- ItemValue is associated to the pugs abstract type item_value

This is done to enrich the data flow between functions but cannot
really be manipulated in the language.

However, one can interpolate to ItemData and can post process
them (only CellValue and NodeValue are supported by VTK and Gnuplot
writers).

- doc and tests have been updated