Interpolation fixes

[nirvn]

Description

Unfortunately, I just discovered we shipped QGIS 3.0 with broken processing interpolation algorithms. This PR fixes that.

@nyalldawson , I'll backport once you OK these fixes.

Checklist

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[nirvn]

I should probably add a note on the ::~:: change. We can't use a comma as separator in our interpolation data widget since memory / temporary scratch layers use comma to define fields, e.g.:

memory?geometry=Point&crs=EPSG:4326&field=StationId:text(0,0)&field=ProviderId:integer(0,0)&field=ProviderName:text(0,0)&field=StationName:text(0,0)&field=Latitude:double(0,0)&field=Longitude:double(0,0)&field=ElevationAboveSeaLevelMeters:double(0,0)&field=DisplayFlag:integer(0,0)&field=temp:double(10,2)

[nyalldawson]

Just goes to prove my view that the most fragile part of processing is now the gui components. I'd love ideas for what we can do to improve this.

But good fix, merge away (and quickly backport for 3.0.2 !)