Remove using 'membersize' attributes in estimators.

**Describe the bug**
In #3809, I found a strange species attribute 'membersize', It was used to identify the number of particles in a given particle group but this should be addressed by `P.groupsize(spec)`.
SpinDensityNew and OneBodyDensityMatrices still depends on it. Once they are fixed, the part in ParticleSet::resetGroups can be cleaned up as well. @PDoakORNL I noticed you already identified its ugliness.