Paper tweak: Improve flow of introduction sentence

[Robinlovelace]

Rephrased "Multi-scale centrality computation is, however, computationally expensive" to "Calculating centrality metrics at multiple scales is computationally expensive, however" for better flow.

[Robinlovelace]

Before:

After:

[songololo]

Hi @Robinlovelace

A heads up that I'm going to close this simply because the sampling paper went in circles a number of times - I tried several non-starters, but I think the current direction may have promise, fingers crossed. It is now completely redrafted, which I have yet to properly review, but if you're interested and if you have feedback, this is the current state:
https://github.com/benchmark-urbanism/cityseer-api/blob/master/analysis/sampling/paper/main.pdf

If you are still interested in co-authoring, we can polish and submit somewhere once we've had a chance to properly review and polish.