Shifting SK PMT seed positions

[pdeperio]

As discussed in #8, need to ensure barrel and end-cap PMTs and bolts are properly shifted along their normal according to the geometry file (i.e. positions nominally correspond to FRP flange/BS plane), in sk_geo_tools.py.

[pdeperio]

Continuing this discussion with @nickwp here:

That's interesting... so does SKDETSIM take this into account when placing the PMTs? If so, can we find out what the assumed distance is from PMT centre as it's placed in SKDETSIM to FRP flange as it's defined in the position file? Is it exactly the 15cm you used as your shift?

I'm not sure what SKDETSIM does with the positions; we need to check.

Here's my best estimate of the physical geometry from overlaying 3 different diagrams of the FRP, PMT, and PMT+dynode (which did not have explicit dimensions so no guarantee that the dynode is in this actual position, since it's secret proprietary Hamamatsu info):