Fix GenEO RHS matrix scaling for blocks

The right hand side matrix of the GenEO eigenproblem is a discretization matrix on the overlap multiplied with the partition of unity from left and right. In the latter, the previous code did not take into account blocking. As a result, eigenvectors and values were wrongly scaled by a factor of block_size. Solver performance was not affected though, the coarse space itself is identical.

Thanks to @peter for finding this!