remove 'mpgPlot' element from gsMPG object

[achubaty]

create a plot.mpg method which plots the raster component (likely this should be default -- user-specified with an argument?).

how to deal with the igraph component?

[achubaty]

I've decided against removing mpgPlot, based on how the mpg object is constructed from the hce object. It's way easier to add the raster here rather than try to reconstruct it after the mpg object is created (since we no longer would have access to the temporary hce object).