Support for efficient concurrent simulation of multiple instances of a model 

Eliminated or reduced the use of global variables in sundials and dassl backends;
Factored out the Sims structure into SimState and SimHistory which are used to represent multiple instantation of a single Sim system of equations;
Added support for Parameter objects, which are represented analogous to states in SimState and can be used to customize the parameters of individual instances;
Added a number of examples of neuronal dynamics;
Initial effort to replace the {} syntax with @equations begin ... end blocks and Equation{}.