Upgrade/include far field current

[peter-madigan]

This includes a number of optimizations to enable the use of a current model that covers the full drift along with 3 neighboring pixels.

• simulate_pixels.py creates a lookup table for event start and end indices to speed up the loop over track segments
• simulate_pixels.py uses a loop when creating the track -> unique pixel map which uses substantially less GPU memory without any noticeable performance loss
• simulate_pixels.py uses a fixed max_radius = 3.5 when finding nearby pixels
• time ticks are now handled in a more coherent fashion under the assumption that only one event is processed at a time, this has a side effect of reducing the GPU memory consumption and allowing for better simulation of time gaps between events
• a new current model response_44-far_field.npy has been added
• a new detector configuration module0-far_field.yaml has been added

Note: this also includes the changes made in the upgrade/add-discriminator-threshold-noise branch -- it might be simpler to just merge this one in rather than trying to do both.