Classical intensities as density

[ddahlbom]

This PR

• Divides the output of intensities_interpolated by sc.Δωso that the returned values are densities. If sc.Δω=NaN (because sc is an instant structure factor) divides by 1 instead.
• Updates tests correspondingly.
• Updates colorranges of heatmap plots in examples.
• Pre-plans additional FFTs for new correlation scheme and eliminates an allocation.

[kbarros]

Looks good to me, pending Sam's comments on binned intensities. Thanks David.

[Lazersmoke]

Hi David, thanks for writing this up.

I think the better way to make this change is to remove the 1/sqrt(n all omega) from the time fft normalization factor. The complete end to end calculation in the time direction is then:

1. For each time offset t, sum up the |t-T| estimates (by fft method), and divide by that number of estimates. This results in a density already! (It's a number that does not go down when the bin size is decreased)
2. For interpolation, just return the density (no factor)
3. For binning, calculate and include the factor (this is done at the end of intensities_binned already, which is why there's a bug happening in the docs)

[Lazersmoke]

(per the discussion around #246 , the overall color scale should have not changed during that, but it turns out it actually did because the aforementioned inclusion of the factor in step 3 happened when that was merged)

[Lazersmoke]

See #273