MD stats files appends by default

[ElliottKasoar]

As part of #230, we append the MD stats header when writing in all cases: https://github.com/stfc/janus-core/blob/main/janus_core/calculations/md.py#L849

This means running the same calculation e.g.

janus md --ensemble nvt --struct tests/data/NaCl.cif --steps 2 --stats-every 1 --traj-every 1 
janus md --ensemble nvt --struct tests/data/NaCl.cif --steps 2 --stats-every 1 --traj-every 1 

creates two sets of headers in the same file:

# Step | Real_Time [s] | Time [fs] | Epot/N [eV] | EKin/N [eV] | T [K] | ETot/N [eV] | Density [g/cm^3] | Volume [Ang^3] | P [GPa] | Pxx [GPa] | Pyy [GPa] | Pzz [GPa] | Pyz [GPa] | Pxz [GPa] | Pxy [GPa] | Target_T [K]
         0 0.252          0.00 -3.37939097e+00 4.25131948e-02 328.897 -3.33687778e+00 2.164 1.79406144e+02 9.68851553e-01 9.32855803e-01 1.03079033e+00 9.42908522e-01 -3.80292164e-02 -2.35372733e-02 1.10649754e-01 300.00000
         1 0.229          1.00 -3.37935448e+00 4.13830343e-02 320.154 -3.33797145e+00 2.164 1.79406144e+02 9.64304008e-01 9.26742310e-01 1.02031498e+00 9.45854731e-01 -1.74348173e-02 -1.07082055e-02 9.71744091e-02 300.00000
         2 0.072          2.00 -3.37924838e+00 3.80714726e-02 294.534 -3.34117691e+00 2.164 1.79406144e+02 9.50926794e-01 9.10987322e-01 1.02354153e+00 9.18251529e-01 -2.87410219e-02 -4.01354358e-03 8.88494687e-02 300.00000
# Step | Real_Time [s] | Time [fs] | Epot/N [eV] | EKin/N [eV] | T [K] | ETot/N [eV] | Density [g/cm^3] | Volume [Ang^3] | P [GPa] | Pxx [GPa] | Pyy [GPa] | Pzz [GPa] | Pyz [GPa] | Pxz [GPa] | Pxy [GPa] | Target_T [K]
         0 0.273          0.00 -3.37939097e+00 3.28890051e-02 254.441 -3.34650197e+00 2.164 1.79406144e+02 9.23012464e-01 1.02846213e+00 8.62384128e-01 8.78191129e-01 5.81712201e-02 -2.37951393e-02 1.51360932e-02 300.00000
         1 0.220          1.00 -3.37935900e+00 3.17246846e-02 245.433 -3.34763432e+00 2.164 1.79406144e+02 9.18364561e-01 1.02950056e+00 8.65033703e-01 8.60559418e-01 5.34354353e-02 -2.80580164e-02 1.27991633e-02 300.00000
         2 0.069          2.00 -3.37926359e+00 3.08789967e-02 238.891 -3.34838459e+00 2.164 1.79406144e+02 9.17023822e-01 1.03640370e+00 8.72749112e-01 8.41918661e-01 4.24732115e-02 -2.35075712e-02 2.43917085e-02 300.00000

I think, if we're writing the header (which may not be the case for restarting, etc.), we usually want to overwrite the file, as we used to?