Enable dsl observed

[TorkelE]

Merges #735 into v14.

Original PR test:

Enables observables to be created as a part of the DSL. Example:

using Catalyst 
rn = @reaction_network begin
  @observables begin
    X ~ Xi + Xa
  end
  (p,d), 0 <--> Xi
  (k1,k2), Xi <--> Xa
end

if we simulate the system, the observables can now be used for indexing:

using OrdinaryDiffEq
@unpack Xi, Xa, X, p, d, k1, k2 = rn
u0 = [Xi => 0.0, Xa => 0.0]
ps = [p => 1.0, d => 0.2, k1 => 1.0, k2 => 2.0]
oprob = ODEProblem(rn, u0, (0.0, 10.0), ps)
sol = solve(oprob, Tsit5())
sol[X]

using Plots
plot(sol; idxs=X)

Some additional notes:

• If only a single observable is used, the begin ... end block is not required:

  @observables X ~ Xi + Xa
  (p,d), 0 <--> Xi
  (k1,k2), Xi <--> Xa
end

• The right-hand side can have any algebraic expression. However, all stuff there must be declared elsewhere (and it cannot contain other observables).
• Currently, the observables do not have any independent variables. We might want to change this part (although it will require a decent amount of coding).
• Metadata for observables is not supported either. This should probably be added I think, but not sure.

Tests and docs are here, so in principle this is ready
(unless we want to support independent variables in this PR)

Consideration regarding independent variables for observables: If the thing it consists of has several different ones, the iv for the observable should be the union of all those occurring in its formula, right?

[isaacsas]

• Can you add a test that when predeclaring a species or variable observable it doesn't end up in states(sys) or species(sys)?
• I wasn't quite clear -- should this be correctly inferring ivs as the union of the rhs terms?
• Can you add a check that interpolation of an external variable/species/parameter works on the lhs of an observable?

[TorkelE]

I will add additional tests. Yes, the ivs of the observables is the same as the ivs of the right hand stuff (the union, using the order with which they appear in the actual reaction network.

fixed requested changes