Fix fuser handling in plugin layer.

[SpacemanPaul]

Since the plugin API refactor, the default fuser function has been broken (it was doing nothing instead of filtering out datasets that returned no data in the region of interest). This was picked up by DEAfrica in their S2 geomad semi-annual runs. This PR (plus corresponding tweaks to the opendatacube/datacube_docker repo, coming soon) should fix.

[codecov]

Codecov Report

Merging #8 (13e9450) into develop (30833ce) will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##           develop       #8      +/-   ##
===========================================
+ Coverage    61.43%   61.45%   +0.02%     
===========================================
  Files           19       19              
  Lines         1815     1816       +1     
===========================================
+ Hits          1115     1116       +1     
  Misses         700      700              

Impacted Files	Coverage Δ
odc/stats/plugins/_base.py	76.92% <50.00%> (+0.60%)	⬆️
odc/stats/plugins/gm.py	68.00% <66.66%> (ø)
odc/stats/_version.py	100.00% <100.00%> (ø)

---

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 30833ce...13e9450. Read the comment docs.