Transport activity enhancements for 2022-W30

[khaeru]

• Remove "tonne_freight" so that e.g. "tkm" is parsed as tonne * km; with ordinary mass units.
• Add [energy] units "litre_of_diesel_equivalent"; partly addresses #24.
• Release as v2022.7.25

[codecov-commenter]

Codecov Report

Merging #39 (5eed573) into main (5b9eebd) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #39   +/-   ##
=======================================
  Coverage   73.10%   73.10%           
=======================================
  Files           4        4           
  Lines         145      145           
=======================================
  Hits          106      106           
  Misses         39       39           

Impacted Files	Coverage Δ
iam_units/test_all.py	100.00% <ø> (ø)
iam_units/emissions.py	100.00% <100.00%> (ø)

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