Magic Number in SampleStack

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SampleStack contains [max_samples_ = 10](https://github.com/QMCPACK/qmcpack/blob/7a3f3da4fa624b02eceb0d78822a0f571adfe575/src/Particle/SampleStack.cpp#L19), without any context for this choice.

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We need to document what `max_samples_ `does and use that info to decide what is a sensible value. We should also not change it just for the sake of changing it.

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If it ain't broke...

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This was found in #4496.

MaxSamples and it's initialization to 10 was introduced in 2cba125 ,
but it does not appear MaxSamples was used for anything in that commit.

`max_samples_` is used in `QMCCostFunctionBatched::correlatedSampling`, see [L461](https://github.com/QMCPACK/qmcpack/blob/7a3f3da4fa624b02eceb0d78822a0f571adfe575/src/QMCDrivers/WFOpt/QMCCostFunctionBatched.cpp#L461), [L548](https://github.com/QMCPACK/qmcpack/blob/7a3f3da4fa624b02eceb0d78822a0f571adfe575/src/QMCDrivers/WFOpt/QMCCostFunctionBatched.cpp#L548) and [L670](https://github.com/QMCPACK/qmcpack/blob/7a3f3da4fa624b02eceb0d78822a0f571adfe575/src/QMCDrivers/WFOpt/QMCCostFunctionBatched.cpp#L670)