`trnorm` performance warnings

[Sergei]

There are two issuses:

1. No preallocation:

    if ndims(T) == 3
        % recurse for transform sequence
        nd = size(T, 3);
        r = zeros(4,4,nd);
        for i=1:nd
            TR(:,:,i) = trnorm(T(:,:,i));
        end
        return
    end

TR is not preallocated as it intended to be (by previous r = zeros(4,4,nd);).

2. Value unused (n at line 63):

    n = T(1:3,1); o = T(1:3,2); a = T(1:3,3);
    n = cross(o, a);         % N = O x A

[Peter Corke]

Thanks for pointing this out, now fixed. Sadly, there is a long list of performance warnings that need attending to. Help and pull requests needed.