(tree files) and fixed a bug when writing to a file and calculating the
offset for the index block for tree files (position would get larger
than 256, which caused an array out of bounds exception).
the physical disk supports two logical disks. Added a logical disk number
which returns 0 when there are no logical disks or the specific logical
disk number if they exist. Currently, only UniDosFormatDisk does this.
In the future, Pascal disk may support this and possibly ProDOS disks
that have sections set aside (ie, a Pascal partition) - if they exist in the
real world.