Give ParticleSet& W a better name

**Is your feature request related to a problem? Please describe.**
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

As discussed in #4127 [ParticleSet& W;](https://github.com/QMCPACK/qmcpack/blob/develop/src/QMCDrivers/WFOpt/QMCCostFunctionBase.h#L177) is a terrible variable name.

**Describe the solution you'd like**
A clear and concise description of what you want to happen.

Change it to something more descriptive. Suggestions? @PDoakORNL @ye-luo

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

Keep the same name, but won't improve readability.

**Additional context**
Add any other context or screenshots about the feature request here.

See #4127