CUDAEnsemble truncate log files.

[Robadob]

Ensemble and simulation now have the argument --truncate or config variable truncate_log_files.

If true, this causes all output files to overwrite existing files (and in the case of ensemble exit logs, it will simply delete files at the start).

If false a FileAlreadyExists exception will be raised. The existence of files is checked at the start of calls to simulate() in the case of ensemble runs to ensure early exit, rather than after waiting for compute. Simulation's however perform this as the time of logging (to avoid repetition, due to those methods being possible to call in isolation).

• CUDAEnsemble
• CUDASimulation
• Tests

Closes #818

[ptheywood]

Looks good, both test suites pass under linux.

I've added 2 tests for --truncate argparsing to python, just to make sure it's available and the config properties can be accessed.