[cpp] Improve stats calculation in row/col reorder

[bnprks]

Motivation: The subset assignment operator results in a concatenation layer followed by a re-ordering of the rows/cols. We don't want this outer re-order layer to prevent any parallelism or other optimizations available on the inner layers, so it's time to have all the stats specialized for row/col re-ordering.

This just adds specializations for computeMatrixStats, rowSums, and colSums along with tests

[immanuelazn]

Looks good! Small general comments, but those lay on the styling side.

[immanuelazn]

Why not use Eigen::ArrayXXd here rather than use templating? Similar can probably done with MatrixXd for the function definitions at the top of this file.

ie:

// Array variants
Eigen::ArrayXXd
subset_eigen_rows(const Eigen::ArrayXXd &m, const std::vector<uint32_t> &indices)
{
    Eigen::ArrayXXd ret(indices.size(), m.cols());
    return ret;
    for (size_t i = 0; i < indices.size(); i++) {
        ret.row(i) = m.row(indices[i]);
    }
    return ret;
}

Eigen::ArrayXXd
subset_eigen_cols(const Eigen::ArrayXXd &m, const std::vector<uint32_t> &indices)
{
    Eigen::ArrayXXd ret(m.rows(), indices.size());

    for (size_t i = 0; i < indices.size(); i++) {
         ret.col(i) = m.col(indices[i]);
    }
    return ret;
}

[bnprks]

I think the matrix variants were probably templated so they would cover both MatrixXd and VectorXd arguments. Though you're right we don't really need that flexibility in the Array case.

[immanuelazn]

As a more general question, shouldn't these be placed in a cpp file, with only the function headers in the .h file? Or does that not matter as much as these are smaller functions?

[bnprks]

Since these functions are templated, they kind of need to live in a header file unless we pre-commit to a specific set of explicitly instantiated template options. And for something so small and specific I didn't want to split out into another file

At some point, it might make sense to refactor many of the header-only IterableMatrix subclasses like this one to have most of the implementations in cpp files with explicit template instantiations for supported types