Add dedicated section on immutability and reproducibility of a graph

[plafer]

We need to share the insight that if packets are treated as immutable, and no global state is used, then a graph's outputs given a set of inputs will always be the same (assuming of course no random number is used insight a calculator).

For example, this example might give different results on different runs because global state is used.

using Tissue;
x = 0

struct Source
    count
    Source() = new(0)
end
function Tissue.process(c::Source)
    c.count += 1
    return c.count > 50 ? nothing : 1
end

struct Adder end
function Tissue.process(c::Adder, num) 
    global x += 1
    return num
end

struct Multer end
function Tissue.process(c::Multer, num) 
    global x *= 2
    return num
end

struct Barrier end
function Tissue.process(c::Barrier, num1, num2) 
    global x
    x
end

@graph NonReproducibleGraph begin
    @calculator source = Source()
    @calculator adder = Adder()
    @calculator multer = Multer()
    @calculator barrier = Barrier()

    @bindstreams adder (num = source)
    @bindstreams multer (num = source)
    @bindstreams barrier (num1 = adder) (num2 = multer)
end

The example for using mutable is very similar: Source sends a mutable struct object with an Atomic{Int} field, and adder and multer modify that field similar to how they modify global x in this example.