Implement some operators for boolean matrices

[mpizenberg]

Today I wanted to do some logical operations on matrices of booleans. Ideally, it would be as simple as calling m1 | m2 & m3 for example. My concrete use case is building the union of masks and I'm currently doing the following (with some additional asserts on sizes):

/// Merge multiple sparse matrices into one combining all sparsely selected pixels.
pub fn merge(matrices: &[DMatrix<bool>]) -> DMatrix<bool> {
    let mut merged = matrices[0].clone();
    for mat in matrices.iter().skip(1) {
        for (b_merged, b) in merged.iter_mut().zip(mat) {
            *b_merged |= b;
        }
    }
    merged
}

[sebcrozet]

We could add component-wise boolean operators, yes.
Note however that you can currently do this:

pub fn merge(matrices: &[DMatrix<bool>]) -> DMatrix<bool> {
    let mut merged = matrices[0].clone();
    for mat in matrices.iter().skip(1) {
        merged.zip_map(mat, |a, b| a | b);
    }
    merged
}

If three matrices are involved, you can do merged.zip_zip_apply(mat, |a, b, c| a | b & c).
There are also zip_map and zip_zip_map methods that do the same except that they return a new matrix instead of modifying self in-place.

[mpizenberg]

Thanks, I think it's best to wait if other people need it to avoid adding stuff to nalgebra that only I use. But if others react to this issue maybe it will be useful. I mostly wanted to open that issue to report this usage and track its interest.