Htg io2 test fix - vtkBitArray initialization of last byte in memory

Test TestXMLHyperTreeGridIO2 was failing on mun because it was writing a vtkBitArray. The failure came from the writer using the whole allocated memory with bit shifts, which had uninitialized bits on the last byte whenever the size of the vtkBitArray was not divisible by 8. Those unused bits that are out of range are set to zero.

This MR also adds a few little fixes in the htg writer and reader.