Add ability to calculate signals on GPU

This adds the ability to calculate signals on the GPU. We use the "old" method of adding directly to the signal arrays rather than doing it after the avalanche as this was the most straight forward. Results look qualitatively similar to the CPU signals. See figure below showing a handful of events where CPU is solid and GPU is dashed. The random numbers are different so we don't expect an exact match.

At the moment the signal can only be calculated using the weighting field and not the weighting potential.

This was probably the main missing feature on the GPU. Looking forward to hearing feedback from users!