fix bug: R'=R*SD and A=R'*R'.T -> R'=SD*R and A=R'.T*R'  when solving for mean spectrum. this has been valided with unit test including large change of resolution among fibers
fix bug: R'=R*SD and A=R'*R'.T -> R'=SD*R and A=R'.T*R'  when solving for mean spectrum. this has been valided with unit test including large change of resolution among fibers

timahutchinson/desispec,profxj/desispec,gdhungana/desispec,profxj/desispec,desihub/desispec,desihub/desispec,timahutchinson/desispec,gdhungana/desispec