siesta as subroutine: fix issue of world_comm and reading the cmd_args

Details

Fixing the issues #408 (closed) and #409 (closed) related to siesta-as-subroutine:

• Introduce an optional logical argument skip_cmdargs into subroutines siesta_init and subroutine reinit. When skip_cmdargs=.true., it avoids reading any command line arguments. When Siesta is launched as subroutine (through siesta_launch), all info is passed through the filename argument (sname); there are no command line arguments to be read, thus the flag skip_cmdargs=.true. is passed to avoid reading the command line. When the argument skip_cmdargs is not present in siesta_init, the default behaviour is preserved, which is to read the command line.
• Improve the code for the case when an mpi comm is passed to siesta_launch.
• Refine examples of this MPI-based API

Why is it needed

• Ability to call siesta as subroutine, with mpi comm that can be smaller than comm_world.
• Prevent Siesta from looking at command-line arguments when running in subroutine mode.

Authors

Miha Gunde and Alberto Garcia

Fixes #408 (closed) Fixes #409 (closed)