Make clear what goes on the right hand side of the Stokes equations, namely the force density due to gravity that depends on the rock density, not a temperature. Also shift the pressure so that it is never negative.
Make clear what goes on the right hand side of the Stokes equations, namely the force density due to gravity that depends on the rock density, not a temperature. Also shift the pressure so that it is never negative.

git-svn-id: 31d9d2f6432a47c86a3640814024c107794ea77c@19444 0785d39b-7218-0410-832d-ea1e28bc413d

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