<pre class="comment"> APCAX  Compute principal components and cumulated virtual coherences 

       [PC,CVCxx,VCxx] = apcax(Gxx)

       PC          Principal components based on Gxx, N/2+1-by-R
       CVCxx       Cumulated virtual coherences between PC's and signals x, N/2+1-by-R-by-R
                   2nd indeces are signals x, 3rd indeces are virtual signals x'
       VCxx        Virtual coherences between PC's and signals, same
                   indeces as for CVCxx

       Gxx         Input cross-spe