build-in support for discrete-event simulation

[jsherfey]

example -- integrate-and-fire model

need support for conditional statements

ex) if (V>=Vthresh) { V=Vreset; g=g+b }

in parse_mech_spec():
add mech.conditional
{ condition, response }

in buildmodel(): do all substitutions into condition and response

in dnsimulator(): fprint after ODE updates:
sel=var(:,k)>parm % i.e., sel=(condition)
if any(sel)
vars(sel,k)=f(vars(sel,:),params) % vars(sel,k)=(response)