bias_patch - trigger_patch mapping

[dneise]

We have 1440 pixel, 320 bias patches and 160 trigger patches. When reading information about them from their particular auxiliary files and/or raw files, we need to assign them somehow to each other.

Knowing that trigger_patch_id = pixel_chid // 9, one might assume that also trigge_patch_id = bias_patch_id // 2. But that is not the case.

So I propose:

The mapping between bias patches and trigger patches must be easily available in pyfact.