TIFFFormat: rename method

prepareToWriteImage should be prepareToWritePlane. Wasn't updated
properly for naming scheme changes from Bio-Formats to SCIFIO.
TIFFFormat: refactor BigTIFF logic

Changed the isBigTIFF flag to be a Boolean, with a null value indicating
the writer should decide whether or not to write BigTIFF data. If the
output dataset is >2GB, BigTIFF is now set automatically.
AbstractMetadata: size calc should be a long

Returned value is a long. Calculation is a sum of longs. Clearly the
local size field should not be an int.
Convert: only set bigTiff if requested

Instead of always setting to "false" by default, only set if the user
actually passed the bigTiff flag.