Add "entity heating cost" variable for space locations

[Kusibu]

TL;DR

New variable on space locations (modifiable at runtime) that scales the heating energy of buildings.

What?

The variable is a multiplier to the baseline heating energy of each building. At a value of 1 (which would be the default), the heating cost of each building will be the same as specified in the prototype; 2 would double the required heat, 0.5 would halve it, et cetera.

This could potentially be set up to entirely replace entities_require_heating (buildings simply don't freeze when the value is at 0, the default) so you don't have two variables doing "the same thing".

Why?

Provides a way for mods to customize freezing on Aquilo or mod-added cold planets in more depth without having to change heating energy on each entity. Runtime accessibility would create more ways that heat can be used by mods, such as a "snowstorm" on a planet that adds or accentuates entity freezing.