Support anisotropic pixels

[ivmartel]

The library should support anisotropic pixels and create a correct canvas representation.

[ivmartel]

Spacing can be found in 4 tags, as mentioned in the Pixel Aspect Ratio definition part03/sect_C.7.6.3.html#table_C.7-11c:

Ratio of the vertical size and horizontal size of the pixels in the image specified by a pair of integer values where the first value is the vertical pixel size, and the second value is the horizontal pixel size. Required if the aspect ratio values do not have a ratio of 1:1 and the physical pixel spacing is not specified by Pixel Spacing (0028,0030), or Imager Pixel Spacing (0018,1164) or Nominal Scanned Pixel Spacing (0018,2010), either for the entire Image or per-frame in a Functional Group Macro.