The beginning of the simulation engine implementation.
Added update functions for various physics parameters.
Split pre_sim_engine into pre_sim_worker and pre_sim_worker_sup.

Also, roughed out the basic implementation of pre_sim_worker and pre_sim_worker_sup, and added pre_sim_worker_sup to the supervision tree.
Removed pre_entity.hrl include, and fixed warning about unsafe 'Else' variable.
Corrected pre_sim_worker and pre_sim_worker_sup so they compile.
Fixed compilation, and greatly simplified entity controllers.
Correctly pattern-match rows returned from the pre_sim_entity_table table.
Actually pass a worker PID to add_entity, and update pre_sim_entity_table correctly.
Corrected specs for external API, and split to_entity into cast and call functions.