qr factorisation method for nullspace

[gszep]

I've opened this feature request based off the back of a thread on slack with @andreasnoack . Since QR is not as reliable as SVD in determining the rank, we could have an additional kwarg where the user provides an expected dimension of the nullspace. Roughly

function nullspace(A::AbstractMatrix; nullity::Int=0, kwargs...)

    if iszero(nullity) # default to SVD
        return nullspace(A;kwargs...)
   
    else 
        return qr(A').Q[:,end+1-nullity:end]
    end
end

A small test

A = randn(3)randn(3)'
nullity = 2 # expected nullity

A*nullspace(A)
A*qr(A').Q[:,end+1-nullity:end]

[andreasnoack]

Ref #514