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.
I know, but that is only relevant for EELSSpectrum, where an automatic background component is generated - that should not take too long, and allows to be general in the code.