RIX BBR: Improve description chop/nod technique

[hugobuddel]

In section 4.2 imaging in LM and N on the sentence

the standard chop/nod technique will be employed for background subtraction

Bernhard Brandl comments:

No, what we do with chop/nod is certainly non standard. Maybe say "and a chop/nod technique, which meets the specific ELT requirements, will be ..."

so we need to describe that better.

[gotten]

See Operational Concept Description v4 (1057) subsection 5.3 for the non-standard aspect.

[hugobuddel]

Updated in #183 with link to Critical Algorithm section about N-band background subtraction, there added a link to the OCD, and also expanded with the information from #144.