ENH: Move Ranks/NumRanks ivars to GenericIOReader
ENH: Update ReadVariableHeader to use EntireHeader

Changed ReadVariableHeader to extrac the desired VariableHeader
from the EntireHeader instead of doing a read in the file.
ENH: Update ReadBlockHeader to use EntireHeader

Changed ReadBlockHeader to extract the desired BlockHeader
from the EntireHeader instead of goind a read to a file.
ENH: Completed OpenAndReadHeader with checksums