`consumedSystemFlux` gets set to zero incorrectly if the heat consumed by a `ModuleSystemHeat` exactly matches the remaining total flux

[Phantomical]

I'm not sure anyone has noticed this in practice, since it requires some specific conditions to actually hit. I've been trying to build some new modules on top of SystemHeat and happened to run into it.

Here's the setup you need to verify this:

• Create a producer A that produces X flux (e.g. 20 MW)
• Create a consumer B that consumes exactly X flux
• Put these both on the same loop
• The ModuleSystemHeat for B will have consumedSystemFlux set to 0 when it should instead be set to -X

The problem seems to be that in this branch in HeatLoop.AllocateFlux which seems like it should really be setting consumedSystemFlux to -totalFlux instead.

[Phantomical]

I have now been poking around some more and have noticed one other case where consumedSystemFlux seems to be inaccurate. If a loop is overheated then consumedSystemFlux is allocated based on the positive flux but doesn't take into account the flux used to reduce the loop temperature.

I will put up a PR to address both of these shortly.