Improve spy for linops

Closes #803.

Separates continuous picture from discrete. The issue of sparsity in the continuous picture is not addressed. 
@tobydriscoll: I don't see how this could have worked?
```
>> D = chebop(@(x,u) diff(u));
>> spy(colloc2(linop(D)))
Error using chebDiscretization/matrix (line 28)
Must specify one dimension value for each subinterval.
Error in chebDiscretization/spy (line 48)
data = matrix(disc); 
```