Re-enable near-null-space vectors use in tentative prolongation
Re-enable near-null-space vectors use in tentative prolongation

This lets non-scalar value types compile, but would probably give
incorrect results. Just a temporary workaround.

dondieselkopf/amgcl,ddemidov/amgcl,ddemidov/amgcl,ddemidov/amgcl,ddemidov/amgcl,dondieselkopf/amgcl,dondieselkopf/amgcl