add interpolating resample filter

[felixmxr]

added resample_int
it interpolates the resampled points for numeric data

[dbuezas]

Thanks, that's a good idea
I think it would be more aligned with the API of the other filters if you make interpolation a parameter of resample.

The integrate filter is a good example of a filter with defaults and either a duration or an object as parameter.
Also describing the filter in the right section of the readme.