Add note that responses should be smooth and well-sampled

[cdeil]

We should add a note to the current IACT DL3 specs that responses are expected to be smooth and well-sampled, i.e. the burden of e.g. smoothing responses and choosing response sampling is on the DL3 data producer, not the science tools consuming the files.

This has been discussed in detail the the April f2f meeting and I think isn't controversial, so "just" needs someone to make this clear in the current spec. But the question came up in another issue (see https://github.com/open-gamma-ray-astro/gamma-astro-data-formats/issues/67#issuecomment-251415391) in a discussion with @kosack , so I'm splitting it out into this separate reminder issue.

General schemes to handle parameter ranges where the responses are bad (like mask or weight arrays) haven't been investigated, but might ultimately be needed. If you'd like to discuss this, I'd suggest to split it out yet again into a separate (very long-term, large effort) separate issue.