LA: Build.SameType of one or two example instances
LA: diagonal-dense Multiply TransoseAndMultiply TransposeThisAndMultiply
LA: diagonal matrix-vector Multiply LeftMultiply/TransposeThisAndMultiply
LA: Diagonal matrices: clean up some pointless null-checks
LA: diagonal-dense Add Subtract
Fit: use diagonal matrices where appropritate (since they are much faster now)