Particle and Track WG 2018 work plan

A) redesigning the unified way of tracking ions, muonic atoms, radicals, electrons/holes/phonons (2)
- working with Tracking WG
B) performance tests and improvements (1)/(2)
- working with corresponding WGs
C) improvements of code readability and documentation (1)/(2)
D) reconsidering production thresholds (2)/2019
 - working with corresponding WGs

(1) - June 2018
(2) - December 2018