Not-a-Number output by write_velocity_field when leading edge aligns with grid.

[hrobeers]

When the leading edge aligns up exactly with a cell edge, write_velocity_field writes a NaN value.
(yellow indicates NaN)

aligned

not-aligned

NaN values are not supported by the ASCII VTK legacy file format.
In order to generate these images I wrote binary support for VTKFieldWriter, I will create a pull request with binary support.

[hrobeers]

This pull request adds binary VTK support:
#5

To reproduce the velocity field, run gmsh-lifting-surface with the field writer constructed as follows:

 VTKFieldWriter field_writer(nstream::BINARY);

And write the field as follows:

field_writer.write_velocity_field(solver, "velocity-field-bin.vtk",
                                  -0.3, 10,
                                  -0.3, 0.3,
                                  -0.2, 4.7,
                                  0.1,0.1,0.1);