Adding virial support

[ndtrung81]

The PR allows virial contribution from the biasing forces from pysages to be added to the LAMMPS engine.

[ndtrung81]

per our discussion (Jun 27):

• FixDLExt should have the virial contribution as an optional argument, not always 1, and can be set from outside
• FixDLExt should provide a function that returns a DLPack tensor that wraps the virial array

[ndtrung81]

@pabloferz can you remind me of how you would like to have the LAMMPSView member variable of FixDLExt initiated from the python side?

I notice that from the pysages side in backends/lammps.py, the Sampler class inherited from FixDLExt the __init__ function

self.view = LAMMPSView(context)

does this mean I need to "export" the member variable from C++ to python as well so that this self.view is actually the same member variable?