Cleaned up.
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Fixed bug for the stress computation in Euler method.
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commit before merge
Merge remote-tracking branch 'refs/remotes/origin/master'
 Bug fixing on Brownian forces
Modified visualization script
Correcting Brownian force correlations
Merge remote-tracking branch 'origin/master'
Commenting out debugging code
Merge remote-tracking branch 'origin/master'
reintroducing ordering
Some temporary codes for 2D test.
Merge remote-tracking branch 'origin/master'
Bug fixing for computation of the resistance coeff when creating and deleting a contact