Enh fancy model indexing

almost everything is passed as a view of the original (an exception is chisq for isig[] indexing, as it is recalculated in this case)

Any suggestions where to add the documentation? Both where model is introduced and fancy signal indexing are possible.
lis = [0,1,2]
for li in lis:
    del li
doesn't work.

The following works
for c in [compo.name for compo in _model]:
     _model.remove(c)