Compute freight volume diagnostic

[khaeru]

This adds an example of one "quality diagnostic" (see #15) calculation involving two input data sets. Namely:

• T003, freight activity, and
• T009, vehicle stock, are used together to compute
• A003, the average freight volume per vehicle per year.

The nomenclature is per earlier scripts developed by @hlinero, and may be changed later.

To do

• [x] Tests and documentation for the new code.
• [x] Complete column infill for T009.
• [x] Ensure the output of this diagnostic is included in the automatically uploaded results.

[codecov[bot]]

Codecov Report

Merging #30 (6b5bcf2) into master (375683f) will increase coverage by 1.00%. The diff coverage is 74.75%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   52.53%   53.53%   +1.00%     
==========================================
  Files          35       38       +3     
  Lines        1521     1582      +61     
==========================================
+ Hits          799      847      +48     
- Misses        722      735      +13     

Impacted Files	Coverage Δ
item/model/plot.py	28.37% <0.00%> (ø)
item/remote/__init__.py	100.00% <ø> (ø)
item/tests/test_historical.py	0.00% <0.00%> (ø)
item/utils.py	100.00% <ø> (ø)
item/historical/diagnostic/__init__.py	48.93% <21.42%> (ø)
item/remote/openkapsarc.py	90.52% <50.00%> (+0.42%)	:arrow_up:
item/historical/scripts/T009.py	93.75% <93.75%> (ø)
item/historical/__init__.py	92.47% <95.45%> (+2.70%)	:arrow_up:
item/historical/diagnostic/A003.py	100.00% <100.00%> (ø)
item/historical/scripts/T000.py	100.00% <100.00%> (ø)
... and 6 more

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