Add attributes to preprocessor.trend and preprocessor.climatology and support saving output

[geek-yang]

We need to add clear attributes to preprocessor.trend and preprocessor.climatology, for instance, indicating the parameters for fitting (we can reuse the repr str from #162). And then users are not needed to infer the name of variables from the file/variable name.

We also want to support the export functionality, which allows the user to save the preprocessor.trend and preprocessor.climatology as (separate?) netCDF files.