Retrieving values from vtk arrays does not respect the datatype.

The following example demonstrates the problem

``` python
from tvtk.api import tvtk
import numpy

data = tvtk.PointData()
data.add_array(numpy.ones(shape=(10,), dtype=int))
temp = data.get_array(0)
print temp # temp is vtkIntArray
print list(temp) # this prints a list of floats!
print temp._vtk_obj.GetValue(1) # this correctly prints an integer
print temp[1] # this prints a float!
temp[0] = temp[1] # this will fail!
```

The failure has been replicated with VTK 6.2 and mayavi 4.4.2. Previous versions (e.g VTK 5.10 with mayavi 4.4.0) will not fail, probably due to some coercion in the vtk python wrappers, but the upcasting is still there.