Patch to newest i-PI version

[armaet]

Some changes for an update to the latest i-PI version. Also added the functionality to sample the (N, V, $\sigma_a = 0$, T)-ensemble, where the cell volume is constrained but cell parameters are allowed to fluctuate.

NVT, NPT and fixed volume NPT all seem to work normally, same goes for PLUMED (through Metadynamics at least). Are there additional features that have to be explicitly tested, which are not in the automatic tests?

[armaet]

I made it such that the pressure is set to zero when the volume is constrained, as it does not make sense otherwise. Currently I included a print statement to make the user aware of this. This should of course also be added to the documentation.

[armaet]

In this commit they restructured the drivers and the run_driver-function is no longer included in the pip installation. The driver now works through PES functions, however as we are using a custom driver function it is still preferable to have access to run_driver. I have opened a commit regarding this in the i-PI repo, but for this moment I have just copied the module.