Remove "unknown" as a cal_type

[jpober]

cal_type unknown is not useful and should probably be removed. Can just use None if initializing a blank object. We can move "unknown" to cal_style.

[bhazelton]

The cal_type keyword controls the shapes of some of the arrays in the object, so I don't think that using None will work well. We implicitly default to the shapes associated with the "gain" type, so I'm inclined to default to that.