Outstanding test work on grid-angle rotations

[pp-mo]

Additional testing needed for minor features + behaviours of the grid_angles and rotate_grid_vectors routines in iris.analysis._grid_angles.

CODE CHANGES

• angles routine should handle non-2d coords with meshgrid

ANGLES TEST ASPECTS

• (cube)
• unrecognised inputs (no shapes)
• missing y + x not cube
• coords
  • not 2d : (support?)
  • different shapes : error (except 1d?)
  • different cube dims (2d : ok for 1d)
  • different cs : error
  • no cs, but not angles, i.e. plain X+Y coords : ??
  • X/Y = coord/not, and not/coord
  • cell_angle_boundpoints unrecognised

ROTATE TEST ASPECTS

• u+v
• u+v with angles
• u+v with angles and method
• Exceptions:
  • angles not angular (bad unit)
  • angles not same dims
• ?possibly~ : NaN angles handling ?

[pp-mo]

Moved all these issues into #3114