HMMER can't read data files that were written on a machine of opposite endianness

[npcarter]

HMMER's data files are sensitive to the endianness of the machine they were written on, and cannot be read on a machine of the other endianness. Currently, the magic numbers at the beginning of the data files are sensitive to endianness, so HMMER will signal an error when directed to read a data file of the opposite endianness, but it would be better if we could just correct for endianness.