Simplify MPI abort code path, make C++ exceptions optional

import select

import re

class MPIAbortException(Exception):

  def __init__(self, message):

    self.message = message

  def __str__(self):

    return repr(self.message)

class lammps(object):

  # detect if Python is using version of mpi4py that can pass a communicator

    if cmd: cmd = cmd.encode()

    self.lib.lammps_command(self.lmp,cmd)

    if self.lib.lammps_has_error(self.lmp):

    if self.uses_exceptions and self.lib.lammps_has_error(self.lmp):

      sb = create_string_buffer(100)

      error_type = self.lib.lammps_get_last_error_message(self.lmp, sb, 100)

      error_msg = sb.value.decode().strip()

      if error_type == 2 and lammps.has_mpi4py_v2 and self.comm != None and self.comm.Get_size() > 1:

        print(error_msg, file=sys.stderr)

        print("Aborting...", file=sys.stderr)

        sys.stderr.flush()

        self.comm.Abort()

      if error_type == 2:

        raise MPIAbortException(error_msg)

      raise Exception(error_msg)

  def extract_global(self,name,type):

    if name: name = name.encode()

    self.lib.lammps_scatter_atoms(self.lmp,name,type,count,data)

  @property

  def uses_exceptions(self):

    try:

      if self.lib.lammps_has_error:

        return True

    except(AttributeError):

      return False

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