move `aggregate` here from Rasters

[rafaqz]

Aggregate uses dims and loci for reseampling an array and its dimesions, and it seems pretty generally useful outside of spatial data.

[bgroenks96]

I've been meaning to comment on this for a while... just kept forgetting.

What's the status on this? How hard would it be to do?

Also, this should be a separate issue, but is there any hope for "smarter" resampling that can have irregular intervals, based on the calendar? e.g. one year is not always 365 days, so the interval should be based on the Ti axis. Maybe there is a way to do this with selectors that I just don't see...?

[rafaqz]

It's really just copying the file across and search and replace on Geo for Dim. Except there is no xxSeries object here, so that has to be split out.

Or copy GeoSeries here as DimSeries as in #278 because its really useful, along with slice and combine methods, and copy all of aggregate here.

I didn't know anyone else cared about this so I never got around to it! Feel free to copy it over too.

[kongdd]

I am also interested about this function. Any plan about the implementation?

[rafaqz]

I have no time. PRs accepted

(We just need to copy it from Rasters.jl. Probably also include my unmerged PR to use symbols instead of Center()/Start() etc)