Make sure integer data arrays can be coregistered

[JanisGailis]

There was a numba typing issue in the 'gridtools' code we had
incorporated, that would prevent the downsampling code from
compiling if a non-float datatype is used. Fix borrowed from upstream.

Closes #770

[JanisGailis]

This is why we should really switch to using upstream 'gridtools' :)

esa-esdl/gridtools@e27bc96

@forman Another note. Do you think we should prevent the users from running coregistration with resampling methods that are not suited for int and bool datatypes, by determining this and raising a ValidationError and suggesting to either select_var or choose a method that makes sense? I myself am on the fence about this one.

EDIT: As usual, I'll merge this in a day, if you won't have found time to attend this!

[codecov-io]

Codecov Report

Merging #775 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #775      +/-   ##
==========================================
- Coverage   76.65%   76.64%   -0.01%     
==========================================
  Files          82       82              
  Lines       12730    12730              
==========================================
- Hits         9758     9757       -1     
- Misses       2972     2973       +1

Impacted Files	Coverage Δ
cate/ops/resampling.py	95.66% <100%> (ø)	⬆️
cate/util/process.py	90.15% <0%> (-0.76%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3762653...c52f7a0. Read the comment docs.

[JanisGailis]

Merged as promised :)