Feature for handling Spaghetti plots

This modification contains code for creating spaghetti plots on horizontal sections. When the 'variable' of a 'horizontalsection' is set to 'multiple members' mode then the spaghetti plot appears in the list of 'rendering' options. One could use the 'contour set' and choose the value/ level for which the spaghetti plot across the ensemble members needs to be created. The 'spaghetti' option can be used to set the colour and the line thickness.