Rework VTK extents for structured grids

[jipolanco]

This simplifies and slightly modifies the behaviour of the extent keyword argument for structured grids:

• extent can no longer be given as a 6-element array [x1, x2, y1, y2, z1, z2]. Instead, it should be passed as a tuple of ranges: (x1:x2, y1:y2, z1:z2).
• extents now expect one-based indexing. That is, x1 = y1 = z1 = 1 by default. These are transformed to zero-based when writing to VTK.
• for image data files, extent is now taken into account for shifting the origin from the input coordinates.

Note that extent is undocumented, and in principle only to be used internally to write parallel formats (#98). Therefore this is not really considered a breaking change.