Allowing different ways to make Chebfun2 nonadaptive.

This allows a Chebfun2 of degree (m,n) to be constructed with the following syntax: 
```
g = chebfun2(f,[m,n]); 
```
and
```
g = chebfun2(f,[m,n],domain); 
```
Right now, a user cannot restrict the rank and the degree.