[reductors] build basis for residual_range one source basis vector at a time

This is needed to implement restricted_to_subbasis .
[reductors] add projected_to_subbasis methods to ResidualOperator and NonProjectedResiudalOperator
[reductors] add 'restricted_to_subbasis' method to StationaryCoerciveEstimator
[demos] add option to select reductor in thermablock demo
[demos] flush stdout after printing '.' in thermalblock demo