Inference: Implement `Conley` Standard Errors

[s3alfisc]

Context

For spatial regression, Conley standard errors are quite popular.

To do

• Implement Conley Standard Errors in numba.
• You can use @kylebutts Rcpp code for reference: link
• See this paper for background reading.
• Implement Unit Tests against fixest's implementation of Conley SEs.