Add defs based on MIT Energy Club fact sheet

[khaeru]

I have this 2007 (!) fact sheet produced by a former member of the MIT Energy Club: UnitsAndConversions.pdf

This suggests several possible conversions that could be incorporated in iam-units to make it more broadly useful:

• Density: conversion from [volume] to [mass] and vice versa. Like GWP, this would require a context and substance identifier.
• Energy content, e.g. Lower Heating Value (LHV): conversion from [mass] to [energy] and vice versa.
  • With density, this also allows conversion from [volume] to [energy].
  • Note that pint already includes some definitions like tonne_of_oil_equivalent = 1e10 * international_calorie = toe; these "_equivalent" would be duplicated somewhat by the new context.
• Emissions factors, prices: these are technology- and location- specific, so probably not appropriate for this package.
• The other conversions on the fact sheet should be supported by pint defaults and thus by iam_units.registry, but could still be added to the test suite. They could also be added in a user-friendly format like a Jupyter notebook/Binder.

[francescolovat]

@khaeru I'm adding the conversion to litres of gasoline-equivalent (Lge) in #27.