Add custom interpolation

This MR calls the underlying interpolation routines in scipy rather than using the scipy interp2d method.

This removes some overheads/safety checking which will make things a little faster for time and distance marginalized runs.

I also removed the part where it does the outer product when two arrays are passed. This was also needed for !856 (merged).

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added Efficiency Utility functions labels

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added 2 commits

• 762568df - 1 commit from branch master
• b25acfa1 - Merge remote-tracking branch 'origin/master' into custom-interpolation

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changed milestone to %1.1.0

merged

mentioned in commit 31439b55

mentioned in merge request !923 (merged)

mentioned in merge request !928 (merged)