algorithms.mesh: WarpPoints interface

An interface to Warp vtk meshes from a displacement field. Uses scipy.map_coordinates interpolation.

PS. We should consider adding the following code:
```
from traits.etsconfig.api import ETSConfig
ETSConfig.toolkit = 'null'
```
at the root of nipype imports. As we don't use the gui support of enthought.traits, the toolkit can be set to null and some errors are prevented.

where does this field come from? different nonlinear registration algorithms store this field in different ways. we should specify which types of fields this tool supports.