Add Stefan-Boltzmann constant

[archisman-panigrahi]

I propose we add the Stefan-Boltzmann's constant to the list of predefined constants.
It comes up very often in many astronomical calculations.

We can perhaps call it either stefan_boltzmann_constant or stefan_constant in numbat, and also denote it as sigma and σ (note: that requires editing the example https://github.com/sharkdp/numbat/blob/8d52de96ca341a21b9316cb86737b466718db9d3/examples/typecheck_error/non_scalar_exponent.nbt to because a dummy variable sigma is used there).

[sharkdp]

Sounds good. Thank you.

(note: that requires editing the example https://github.com/sharkdp/numbat/blob/8d52de96ca341a21b9316cb86737b466718db9d3/examples/typecheck_error/non_scalar_exponent.nbt to because a dummy variable sigma is used there).

Maybe we can refrain from using sigma/σ for now? and just add stefan_boltzmann_constant?

[archisman-panigrahi]

Maybe we can refrain from using sigma/σ for now? and just add stefan_boltzmann_constant?

That's fine 👍