Novel features and GPU performance analysis for EM particle transport in the Celeritas code

<!--HTML-->Celeritas is a new computational transport code designed for high-performance simulation of high-energy physics detectors. This work describes some of its current capabilities and the design choices that enable the rapid development of efficient on-device physics. The abstractions that underpin the code design facilitate low-level performance tweaks that require no changes to the higher-level physics code. We evaluate a set of independent changes that together yield an almost 40\% speedup over the original GPU code for a net performance increase of $220\times$ for a single GPU over a single CPU running 8.4M tracks on a small demonstration physics app.