Added Sum resampling method to raster.resampling

[Charmatzis]

Does what it says: Sum method has been added for resampling.

Now the aggregate resampling methods are:

1. Median
2. Max
3. Min
4. Sum

[pomadchin]

Thanks @Charmatzis! Can you sign your commits?
git commit --amend -s && git push origin -f master

[echeipesh]

@Charmatzis actually just adding Signed-off-by: Christos Charmatzis <xr_xarmatzis@outlook.com> to the bottom of the PR is sufficient to make ip-validation happy. I know this is silly, but if you don't mind.

[pomadchin]

@Charmatzis try to rebase on master, probably IP issue with noreply@github.com would go:

git commit rebase -i upstream/master && git push origin -f master

@echeipesh there is a valid sign-off-by footer, looks like IP checker result is that email has no valid ECAs. Can you check it manually?

[echeipesh]

Looks good, blaming ip-validation bot glitch.