[x] document that negative entries will be saved as some implementation-defined negative value, so to_dense may not recover the exact negative value from before.
[x] redo representation slightly for better cache
[x] smart cov calculation with optimization for when i == j case.
[x] change order of operation for muls to be category then columns.
[x] check calldata only contains 0/1 (no missing values!) and ancestries has values [0, A).
nnm(j)to return number of non-missing values for each column.imputed_value(j)to return imputed value for each column.writeshould take in impute method.compute_column_mean,compute_nnm,compute_nnz.to_densemay not recover the exact negative value from before.