PICTFormat: do not add X and Y twice

For version 2 PICTFormat datasets, the X and Y axes could be added
twice, resulting in datasets with multiple X and Y axes.

Updated the parsing to only set axis lengths once.
TIFFFormat: expose BigTiff configuration

It is now possible to set the isBigTiff flag in the TIFFFormat$Writer
through SCIFIOConfig key:value pairs.

Also exposed the isBigTiff flag via an accessor.