gauge writer and checksum

[clarkedavida]

We can now read in NERSC configurations, which is nice, but it would be better if we could write them. For that we need the NERSC checksum from here, but I have to say I don't understand what it's doing.

[clarkedavida]

Could also be nice to have an ILDG writer, since there is now an ILDG reader. Should probably do NERSC first since that is a lot easier.