Save stress tensor in extXYZ file

[AugustinLu]

Close #378

Add stress tensor in the comments of trajectory.xyz (unit: eV/ang**3).

This information can be read by other packages such as ASE.

Example below with a 8-atom Si cell (NPT).

Conquest_out:

      force: Maximum force :        0.05000631(Ha/a0) on atom         5 in z direction
      force: Force Residual:        0.04902342 Ha/a0
      force: Total stress:          0.74155321    -1.84331297     0.12191144 GPa

    MD step:     99 KE:         0.01448494 Ha IntEnergy:         -33.60117595 Ha TotalEnergy:         -33.58669101 Ha

      force: Maximum force :        0.04882996(Ha/a0) on atom         5 in z direction
      force: Force Residual:        0.04893222 Ha/a0
      force: Total stress:          1.38235123    -1.29514909     0.50513119 GPa

    MD step:    100 KE:         0.01443590 Ha IntEnergy:         -33.60778528 Ha TotalEnergy:         -33.59334938 Ha

trajectory.xyz:

8
config_type= Lattice="5.50541532     0.00000000     0.00000000     0.00000000     5.50541532     0.00000000     0.00000000     0.00000000     5.50541532" Properties=species:S:1:pos:R:3:forces:R:3 potential_energy=-914.33456941 pbc="T T T" stress="      0.00462841     0.00000000     0.00000000     0.00000000    -0.01150505     0.00000000     0.00000000     0.00000000     0.00076091"
  Si      5.03110541      5.38506756      5.22845718 -0.18617114E+01 -0.15170907E+01 -0.22133260E+01
  Si      2.75870416      2.76797928      5.46515464  0.90226801E+00  0.56898130E+00 -0.34902481E+00
  Si      2.89712039      5.19585901      2.91043075  0.17172608E+01 -0.19579072E+01  0.21126897E+01
  Si      0.08885933      2.72016595      2.70152288  0.95996331E+00  0.89109100E+00  0.67559888E+00
  Si      1.94790352      1.59155287      1.75630426  0.20379204E+01  0.14600916E+01  0.25714279E+01
  Si      4.04939920      3.95739064      1.41530062 -0.88250661E+00 -0.34564481E+00 -0.21941374E+00
  Si      1.31430491      4.15050224      4.17396457 -0.10978230E+01 -0.73287886E+00 -0.67533129E+00
  Si      3.88675248      1.70319388      3.80414078 -0.17644754E+01  0.16474557E+01 -0.19040854E+01
8
config_type= Lattice="5.51685178     0.00000000     0.00000000     0.00000000     5.51685178     0.00000000     0.00000000     0.00000000     5.51685178" Properties=species:S:1:pos:R:3:forces:R:3 potential_energy=-914.51441853 pbc="T T T" stress="      0.00862796     0.00000000     0.00000000     0.00000000    -0.00808368     0.00000000     0.00000000     0.00000000     0.00315278"
  Si      5.02731627      5.38223528      5.22412773 -0.18484208E+01 -0.16113975E+01 -0.22694932E+01
  Si      2.76083964      2.76925074      5.46510336  0.91441185E+00  0.59975061E+00 -0.34188339E+00
  Si      2.90065330      5.19179413      2.91461982  0.17488271E+01 -0.18838251E+01  0.21483447E+01
  Si      0.09139527      2.72223874      2.70287580  0.82127641E+00  0.84836310E+00  0.70325492E+00
  Si      1.95232365      1.59491703      1.76193152  0.20625112E+01  0.14495057E+01  0.25109372E+01
  Si      4.04706679      3.95624148      1.41411734 -0.85230660E+00 -0.37332125E+00 -0.16801157E+00
  Si      1.31155174      4.14848008      4.17228965 -0.11107098E+01 -0.65453118E+00 -0.64311793E+00
  Si      3.88300462      1.70655638      3.80021020 -0.17270122E+01  0.16197030E+01 -0.19561292E+01