Fix getResolutionDescriptions for pyramid-based files

The dummy implementation from the ROMIO pixel buffer and
the in-memory pixel buffer was also copied into the
BfPyramidPixelBuffer implementation causing the return
values of getResolutionLevels and getResolutionDescriptions
to be out of sync. BfPPB now delegates to the internal BfPB
for the descriptions.
Merge pull request #1016 from joshmoore/pyramid-resolution-descriptions

Fix getResolutionDescriptions for pyramid-based files