Fix chebfun vscale

This "fixes" #895.

The displayed vscale of a chebfun takes into account the point values of f, and this is equivalent to 
```
vscale(f, 'sup')
```
Internally, we use vscale for tolerances and there we ignore values of f at break points while computing its vscale. This is and was the default behaviour when we typed

```
vscale(f)
```
which is the same as
```
vscale(f, 'ess-sup')
```
In particular, the example given in #895  now works:

```
>> f = chebfun(0);
f.pointValues(1) = 1
f =
   chebfun column (1 smooth piece)
       interval       length   endpoint values  
[      -1,       1]        1         0        0 
Epslevel = 2.220446e-16.  Vscale = 1.
>> max(f)
ans =
     1
```