Make variableset real for ray

[jptowns]

Incorporated changes to VariableSet. Now it's RealType and tests pass on QMC_COMPLEX=1 build.

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Proposed changes

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What type(s) of changes does this code introduce?

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• Bugfix
• New feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Testing changes (e.g. new unit/integration/performance tests)
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Does this introduce a breaking change?

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Checklist

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• Yes/No. This PR is up to date with current the current state of 'develop'
• Yes/No. Code added or changed in the PR has been clang-formatted
• Yes/No. This PR adds tests to cover any new code, or to catch a bug that is being fixed
• Yes/No. Documentation has been added (if appropriate)