Preliminary support for Ventana bif format.
Do not have official specs for file format, so this is mostly reverse engineering.

Properties:
-- Pyramidal TIFF file
-- Requires bigtiff
-- Tiles have (seemingly arbitrary) overlap

What this commit includes:
-- Identifies bif file can be opened
-- Parses metadata and populates openslide struct as appropriate.

What this commit does NOT include:
-- Accurate importing of image regions (due to non-standard overlap)