New feature: support timedelta as limit when using interpolate method=time
When we call the interpolate function based on time, it would be useful to be able to give a limit expressed as a timedelta (instead of a number of row).

Use case
- I've a time series measurement of temperature in function of time. The measurements are not performed at a constant frequency
- I want to interpolate missing data if the interpolation period is less than 1 hour.
