Add a vtkMath method to compute the angle between two vectors

Use the atan2 method rather than the seemingly simpler acos(dot) method for
better precision near 0. This method should be robust for vector inputs of
any magnitude, including 0.

Thanks to David Gobbi for the inspiring discussion on the mailing list:
http://vtk.1045678.n5.nabble.com/Angle-between-two-vectors-td5725202.html

Change-Id: I712ca46a83a12e8e424ab05521176ff545c67610
Merge topic 'add-angle-between-vectors-method' into master

80498e4c Add a vtkMath method to compute the angle between two vectors
COMP: Test driver array bounds error

Some compilers report "array subscript is above array bounds" while
compiling vtkRenderingMatplotlibCxxTests. This occurs when
Module_vtkRenderingMatplotlib:BOOL=ON but Matplotlib is not found or
VTK_GHOSTSCRIPT_EXECUTABLE is not defined. In this case, there are no
tests defined, but the test driver is still built.

Rather than change CMake, this patch adds an informative dummy test.

Change-Id: I3f1d6ed2453d33d113f165fc9552f18d2cc4906f
Merge topic 'MatplotLibTest_Warning' into master

2808d60d COMP: Test driver array bounds error