Smoother reentry heat; multivessel

I've tweaked the heat calculations quite a bit, mostly making the heating gradual instead of sudden for a higher tension experience. 0-7 degree reentry is survivable, ~15 degree for pods and heat shields. Ferram planes will nearly *require* lift devices to survive, as they really should.

Try out the new curves; let me know what you think.

I've also added multivessel heating, which is important for when deadly reentry causes "unintended undocking" of everything. :-)