Updated usage of -gpu flag.

README.rst

@@ -169,11 +169,11 @@ When you are finished using gprMax, the conda environment can be deactivated usi
 Optional command line arguments
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 Argument name          Type      Description
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 ``-n``                 integer   number of times to run the input file. This option can be used to run a series of models, e.g. to create a B-scan with 60 traces: ``(gprMax)$ python -m gprMax user_models/cylinder_Bscan_2D.in -n 60``
-``-gpu``               integer NVIDIA CUDA device ID for a specific GPU card. If not specified will default to device ID 0.
+``-gpu``               flag/list flag to use NVIDIA GPU or list of NVIDIA GPU device ID(s) for specific GPU card(s).
 ``-restart``           integer   model number to start/restart simulation from. It would typically be used to restart a series of models from a specific model number, with the ``-n`` argument, e.g. to restart from A-scan 45 when creating a B-scan with 60 traces: ``(gprMax)$ python -m gprMax user_models/cylinder_Bscan_2D.in -n 15 -restart 45``
 ``-task``              integer   task identifier (model number) when running simulation as a job array on `Open Grid Scheduler/Grid Engine <http://gridscheduler.sourceforge.net/index.html>`_. For further details see the `parallel performance section of the User Guide <http://docs.gprmax.com/en/latest/openmp_mpi.html>`_
 ``-mpi``               integer   number of Message Passing Interface (MPI) tasks, i.e. master + workers, for MPI task farm. This option is most usefully combined with ``-n`` to allow individual models to be farmed out using a MPI task farm, e.g. to create a B-scan with 60 traces and use MPI to farm out each trace: ``(gprMax)$ python -m gprMax user_models/cylinder_Bscan_2D.in -n 60 -mpi 61``. For further details see the `parallel performance section of the User Guide <http://docs.gprmax.com/en/latest/openmp_mpi.html>`_
@@ -183,7 +183,7 @@ Argument name          Type    Description
 ``--opt-taguchi``      flag      run a series of models using an optimisation process based on Taguchi's method. For further details see the `user libraries section of the User Guide <http://docs.gprmax.com/en/latest/user_libs_opt_taguchi.html>`_
 ``--write-processed``  flag      write another input file after any Python code and include commands in the original input file have been processed. Useful for checking that any Python code is being correctly processed into gprMax commands.
 ``-h`` or ``--help``   flag      used to get help on command line options.
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 Updating gprMax
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