WIP: add discrete_events to DSL

[tstrey]

Adds @discrete_events option to the DSL such that reaction_system can be can be written in the following way:

rn = @reaction_network rxs begin
     @variables t
     @parameters k
     @species A(t) B(t)
     @discrete_events begin
          (t == 2.0) => [B ~ 0.0]
     end
     k, A --> B
end

[tstrey]

Needs to be modified for multiple discrete events.

[isaacsas]

@TorkelE ended up adding this as part of Catalyst 14, but thanks for your getting this started!