replace use of vtkCellLocator by vtkStaticCellLocator
vtkStaticCellLocator is faster and thread-safe (if BuildLocator is called first in single thread), which makes vtkImplicitPolyDataDistance thread-safe
Other fixes include:
- use vtkIdType for point ids
- fix bug in SetInput: because the input was always passed through the vtkTriangleFilter, the check "if (this->Input == input) was always false.