Update Howto_pylammps.rst

.. code-block:: Python

   L.runs[0].step # list of time steps in first run

   L.runs[0].ke   # list of kinetic energy values in first run

   L.runs[0].thermo.Step # list of time steps in first run

   L.runs[0].thermo.Ke   # list of kinetic energy values in first run

Together with matplotlib plotting data out of LAMMPS becomes simple:

import matplotlib.plot as plt

.. code-block:: Python

   steps = L.runs[0].step

   ke    = L.runs[0].ke

   import matplotlib.plot as plt

   steps = L.runs[0].thermo.Step

   ke    = L.runs[0].thermo.Ke

   plt.plot(steps, ke)

Error handling with PyLammps