Fix Python 3 compatibility by encoding strings passed as c_char_p

  # send a list of commands

  def commands_list(self,cmdlist):

    args = (c_char_p * len(cmdlist))(*cmdlist)

    cmds = [x.encode() for x in cmdlist if type(x) is str]

    args = (c_char_p * len(cmdlist))(*cmds)

    self.lib.lammps_commands_list(self.lmp,len(cmdlist),args)

  # send a string of commands

  def commands_string(self,multicmd):

    if type(multicmd) is str:

        multicmd = multicmd.encode()

    self.lib.lammps_commands_string(self.lmp,c_char_p(multicmd))

  # extract global info