Fix the sign of the heat flux for DISMF

[xylar]

😭😭😭😭😭

Checklist

• [x] Document (in a comment titled Testing in this PR) any testing that was used to verify the changes

[xylar]

Testing

I ran this on the Icos mesh and I'm seeing:

Area of a cell (m^2):             3686400.0
Total flux on plane (kg/s):       30023072.2
Total flux on sphere (kg/s):      30023072.2
Total heat flux on sphere (W):    -10018699206457.0

(I've since changed this to TW for readability.) This seems like the right sign and size of the heat flux, given the mass flux.

[xylar]

I updated the DISMF files for Icos30, SORRM and RRS.