Add tests to pixelflow

[IFenton]

• [x] Check a mask with no objects produces the expected 'None'
• [x] Check scaling works as expected
  • [x] Isotropic scaling
  • [x] Anisotropic scaling
  • [x] No scaling
  • [x] All 1
• [x] Check label option works as expected
• [x] Check features works as expected
  • [x] subsetting
  • [x] 3D subsetting
  • [x] None 2D
  • [x] None 3D
• [x] Check dimension guessing works as expected
  • [x] Error if dimension labels doesn’t match mask dim
• [x] Check img_coords works as expected