ImgSaver: refactor to allow additional img types

This removes the tight integration of ImgSaver with PlanarImg.

The class has been generalized and now relies on two utility methods -
getting a plane count, and extracting an array for a given plane index.
These methods can now be extended to support additional Img types.

For future plans regarding Img compatibility, see:
https://github.com/scifio/scifio/issues/224
ImgSaver: add ArrayImg support

We can now write out ArrayImgs.
Add ConvertImg test

Like ReadImg, but will also save the img with ImgSaver.

Useful for testing writing with non-PLANAR Img types.
ImgSaver: add general case

ImgSaver now has a generalized routine for iterating over output
ImgPluses. In theory it should now be able to support any output image
types (tested with ArrayImg, PlanarImg and CellImg).
Merge branch 'write-nonplanar-types'

Adds support to ImgSaver for writing out non-PlanarImg types.

In particular, there is now a specialized ArrayImg case, and a
generalized fallback case that simply uses cursor iteration.

Also adds a manual test for writing with ImgSaver (ConvertImg).

Closes #175