Decompose unit conversion functions to exact coefficients (breaking)

[lipchev]

This is a port of #1393, targeting the release-v6 consisting of the changes that broke one test or another.

Most of these are related to conversions involving the US/British units but there is also some confusion regarding the Calorie - the value of 4.1868 was used (instead of 4.184) in a few places, which is a note-worthy difference.

Here are all the tests that broke:

• HeatFlux : due to the change in Calorie
• HeatTransferCoefficient : due to the change in Calorie
• Impulse : PoundFootPerSecond, SlugFootPerSecond were represented by a single coefficient that is significantly different from what got: see Decomposed the rounded expressions in the unit definitions into operations with exact coefficients #1393 (comment)
• Luminosity : see Decomposed the rounded expressions in the unit definitions into operations with exact coefficients #1393 (comment).
• Power: MechanicalHorsepower changed from {x} * 745.69 to {x} * 76.0402249 * 9.80665 as given by this article
• Pressure the TechnicalAtmosphere coefficient (no idea where the old one was taken from), see also Decomposed the rounded expressions in the unit definitions into operations with exact coefficients #1393 (comment)
• SpecificFuelConsumption : I'm not a domain expert, but {x} * 28.33 doesn't look right for PoundMassPerPoundForceHour (lb/(lbf·h))- here's what I got: {x} * 453.59237 / (0.0044482216152605 * 3600)
• ThermalResistance: using different conversion coefficients for BTU and Calorie from the one defined in Energy (note: I wasn't able to confirm the "correct" conversion coefficient for BTU).
• VolumetricHeatCapacity: using different conversion coefficient for Calorie

[lipchev]

I still want to replace the PressureUnit.FeetOfElevation and PressureUnit.MeterOfElevation with properties that convert to/from Length (see #1393 (comment)) and remove the Frequency.BUnit but I'll open separate PRs for these (didn't want to have the Units.g.json changed here).

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[angularsen]

Not through everything yet but posting the review so far.
I got through all the JSON files.

[lipchev]

@angularsen I've merged the modifications from release/v6, this should be ready to review- please tell me if there is something else you want me to do.

I'd really like to close this as soon as possible- it's blocking my 🐲 PR..

[angularsen]

I'll make an honest attempt the next few days, I know I'm running late on a bunch of PRs lately and it's on my conscience, I just struggle to find the time and energy.