Merge lp://staging/~fluidity-core/fluidity/speedup-vtudiff into lp://staging/fluidity

This speeds up the vtudiff script run on vtus with multiple fields, by only setting up the VTK probe once instead of once per field. It does not change anything in the way fields are probed.

There is also a little bit of cleaning up:
* removing the option to call vtktools as a main: this really doesn't make sense and should be in a separate script
* removing the "field manipulation" methods. These were basically vectorial operations on fields but implemented in a rather inefficient way. I see no reason to not simply use numpy vector operations for that, so instead of:

vtu.SubFieldFromField("fieldName", array, "newFieldName")

just do:

field = vtu.GetField("fieldName")
vtu.AddField("newFieldName", field-array)