Adding squeeze

Would be nice to have a wrapper for squeeze.

Without squeeze, matmul usage is quite limited because if will fail for all slices of bigger matrices. Example:

```
import arrayfire as af
x = af.constant(0, 10, 5, 5, dtype=af.Dtype.f64)
af.matmul(x[0,:,:],x[1,:,:]) # will fail because only matrices with two dimensions are allowed
```