Add litre gasoline-equivalent conversion

[francescolovat]

This PR addresses the addition of the energy unit conversion of litres of gasoline-equivalent (Lge). These units are used to represent quantities in this GFEI report, from which we happened to received some data.

How to review:

• Check that the following commands work in your own machine:

  from iam_units import registry
  qty = registry('33.5 MJ')
  qty.to('Lge')
  1.0 <Unit('Lge')>

[codecov-commenter]

Codecov Report

Merging #27 (ea8d49c) into main (9d709e9) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #27   +/-   ##
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  Coverage   73.07%   73.07%           
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  Files           4        4           
  Lines         130      130           
=======================================
  Hits           95       95           
  Misses         35       35           

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[francescolovat]

@khaeru thanks! I've just added it. Please feel free to merge it when tests will pass!

[khaeru]

@francescolovat sorry, but I realized belatedly that definitions.txt is organized in sections and this, being energy-related, should have gone in the "Energy" section, rather than at the very bottom of the file.

I will move the lines as I prepare a new release.

[francescolovat]

Thanks for making it even clearer, @khaeru !