Remove X as default manifold

[bosonbaas]

Currently, the Decapodes2D presentation already defines a Space type X. If we expand to allowing simulation over multiple manifolds, we'll want a syntax that allows the user to define what the manifold names are conveniently, without enforcing a default manifold X.

Potentially could use parameters to the macro:

@decapode Decapodes2D(X, Y) begin

end