[factorization] The force data of a new simulation will be appended to existing forces.txt

[piyueh]

In the decoupled flow solver in factorization branch, if we launch a new simulation in a folder with existing forces.txt, new data will be appended to the old data.

1. For a new simulation, the proper behavior should be deleting old forces.txt and creating a new one.
2. If the simulation is in restart mode, then the data will be appended.

[piyueh]

Update:
Now every new simulation will just create a new forces.txt, instead of appending to existing forces.txt. But when we are undergoing "restart" process, this will also overwrite the forces.txt for those prior the restarting point.