Feature request: read deeptile with InputPart/InputFile

Imf::InputFile (and Imf::InputPart) can now read and convert SCANLINEIMAGE, TILEDIMAGE, and DEEPSCANLINE. All that's left to add is DEEPTILE, please!

I guess the first step in that would be to get DeepScanLineInputFile to convert a tiled file if it can't already.

Start by inspecting Imf::InputFile, Imf::InputPart, and DeepScanLineInputFile to determine how the existing SCANLINEIMAGE, TILEDIMAGE, and DEEPSCANLINE conversions work. Establish whether DeepScanLineInputFile already converts tiled files, then define and verify the behavior needed for DEEPTILE support.

Written by the indexing model from the issue text.