allow more config to be unspecified in yaml

I was setting up a northing analysis today and tried to run with a yaml missing the below three settings, thinking they would just use suitable defaults. The analysis failed with a pydantic validation error unless these settings were populated in my yaml. It would be nice to not have to specify these in every yaml because they are rarely set to a non-default value.

• years_offset_for_pre_period (default should be 1)
• ws_bin_width (default should be 1)
• years_for_detrend (default should be 3)