Corrected some syntax for selecting GPU/device with MPI

gprMax/contexts.py

@@ -151,8 +151,8 @@ class MPIContext(Context):
 
         # Check GPU resources versus number of MPI tasks
         if executor.is_master():
-            if config.sim_config.general['cuda']:
-                if executor.size - 1 > len(config.sim_config.cuda['gpus']):
+            if config.sim_config.general['solver'] == 'cuda':
+                if executor.size - 1 > len(config.sim_config.devices['devs']):
                     logger.exception('Not enough GPU resources for number of '
                                      'MPI tasks requested. Number of MPI tasks '
                                      'should be equal to number of GPUs + 1.')