Allow user to provide a stress matrix in the input file for anisotropic barostat

Anisotropic barostat like [`AnisoBenrendsenBarostat`](https://github.com/lumol-org/lumol/blob/0c83518814ccae862dc810023e49ff614049d831/src/core/src/sim/md/integrators.rs#L218) can be constructed with either an isotropic (hydrostatic) pressure, or a full stress matrix.

We only expose the first way in the input, also having the second one might be desirable.