Add min/max for 1d WCS

This implements `min`/`max` for 1D lookup tables.  `min`/`max` should be implemented for all wcs types, but I don't know how at the moment.  I think the polynomial and linear varieties both can be called for *any* pixel, which is good, but it should also be possible to define them with a domain - any given spectrum clearly has a minimum and maximum value.

@wkerzendorf - I could use your help on this.  I need these implemented for a module I'm writing.