initial commit of CHEBOP/DISPLAY method.

```
>> N = chebop(@(x, u,v) [diff(u, 2) + v ; diff(v,2) - u], [-1 1], @(x, u, v) sum(u) )
N =
   Linear operator:
      [diff(u,2)+v;diff(v,2)-u] = 0
   operating on chebfun objects defined on:
      [-1  1]
   with
    constraints:
      sum(u) = 0
>> N.lbc = @(u,v) u - 2
N =
   Linear operator:
      [diff(u,2)+v;diff(v,2)-u] = 0
   operating on chebfun objects defined on:
      [-1  1]
   with
    left boundary conditions:
      u-2 = 0
    constraints:
      sum(u) = 0
```
Improvemnets to formatting.
Removing typo.
Improvements to help text
Merge pull request #942 from chebfun/feature-chebop-display

Feature chebop display