Support event() for global variables

[frothga]

In general, a global variable on LHS does not necessarily mean the expression should be evaluated at the population level. If the RHS references local variables, the equation should be evaluated within a local instance, and perhaps combine results in the population part as if by external reference.

This issue is specifically about population-level equations, rather than instance-level equations that happen to update population-level variables. At present there is no mechanism (in Internal or C) to trigger a population-level event.