Give ParticleSet& W a better name

[quantumsteve]

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As discussed in #4127 ParticleSet& W; is a terrible variable name.

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

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

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See #4127

[PDoakORNL]

walker_configurations_ I think is descriptive of what this particle set is doing. I think. @markdewing should weight in.

ParticleSet is a garbage bag that can contain anything from just a set of positions to a set of positions, or positions and some energy values or... I believe it or its subtypes can also have quite complex historical state contained in them.

As a result the type itself is almost worthless in terms of reasoning about what an instance of it might be for or be able to do.

[markdewing]

The value of ParticleSet& W; declared in QMCCostFunctionBase is used for two purposes:

1. Get the number of particles for allocating temporary storage.
2. Create particle resources in DriverWalkerResourceCollection in the CostFunctionCrowdData constructor.

The uses are only concerned with a single particle set, not a set of configurations. It could be called P, though single letter names are hard to search for. Maybe 'pset_' or 'elec_'?

The other uses of W in files in WFOpt are from the declaration inherited QMCDriver.