Adding a deal method for @chebmatrix.

Closes #1011.

`deal` for chebmatrices does _not_ behave in the same way as MATLAB `deal`, although the idea is the same. Chebmatrix `deal` takes a single input (a chebmatrix) and returns its entries as outputs. (In MATLAB `deal`, `N` inputs are assigned to `N` outputs.) The reason for this deviation is to allow a convenient syntax for getting the solution components of differential equations:
```
N = chebop(@(x,u,v) [diff(u) + v; diff(v) - u], [0,1]);
N.bc = @(x,u,v) [u(0) - 1; v(0)];
[u,v] = deal(N \ 0);  % solution components of a system
                      % of differential equations
```

One point that is up for discussion with the team is my choice of row-major ordering for chebmatrices. We usually follow MATLAB conventions, but in the case of chebmatrices, my experience has shown that row ordering is more important than column ordering (since in a block operator each row corresponds to an equation). I won’t argue too much if other disagree, however. The primary use of `deal` will be in assigning solution components of ODE systems, in which case the chebmatrix is one row of chebfun columns.