Created function update_edge_scales
implemented update_edge_scales in msn functions

poppr.msn, bruvo.msn, singlepop_msn, and plot_poppr_msn were all modified to use this function.
updated plot_poppr_msn man page and NEWS
Removed unnecessary comments
update
changed mlg.group to mlg.id

This seemed to make the function's purpose clearer.
make sure tests work