Improve VR's memory usage for extra data in Particles

[rtobar]

As explained in #112 (comment), VR currently uses an unnecessarily high amount of memory keeping track of extra data associated to each Particle. This is mainly to a std::map with a std::string key being used as the basis of the data structures holding this extra information, and thus the infrastructure-to-data memory usage ratio is probably too high. On top of that, most of the strings in those maps are probably copies of each other, and therefore a non-duplication strategy could be studied.

This ticket is to keep track of the desire of studying this in further detail, and come up with some solution that could replace the current infrastructure with a lower memory footprint (and probably better performance as well).