CSS dtype additions

Add support for CSS file with data in little-endian binary format, as well as ASCII. I don't have any test data for the ASCII format, so hopefully someone will magically find one to test...

There's also a "NORESS gain-ranged" type, but I don't know what that means, so I didn't add it.

PS, the full CSS schema is available on [the Pisces repo](https://github.com/jkmacc-LANL/pisces/blob/dev/docs/data/Anderson1990.pdf).