Sale price spatial lag configuration

[connorschwartz]

The number of nearest neighbors for spatial lag calculations is configurable for other variables under process.enrich.spatial_lag.fields. Spatial lag for sale price is calculated with different code that currently is not configurable. This PR updates the code so that the sale price spatial lag calculation can be adjusted in a similar way.

Also, if k is less than the number of sales in the training set, the calculation fails. So, we can instead use the minimum of k and the length of the training set.

[github-actions]

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