Multimat: update benchmark code

[publixsubfan]

Summary

• Use templates to consolidate direct-access and submap-access microbenchmarks in Multimat calculate.cpp example
  • Adds an enum class MMFieldMethod to test different methods of accessing Multimat field data
  • Adds a FieldGetter helper class which is specialized on MMFieldMethod to encapsulate returning different types of fields
  • This should resolve some inconsistencies seen in results for direct-access microbenchmarks [*]
• Adds corresponding test result outputs to Result_Store helper class; this fixes an issue where the original templated tests would overwrite each other

Removing the other test methods will be deferred for a later PR, since it seems to mess up the diff.

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[*] One of the (faster) examples constructed a ProductSet<RangeSet, RangeSet> to compute the flat index itself, while other methods were calling BivariateMap::findValue(firstIdx, secondIdx). It seems the speedup resulted from the use of slam::RangeSet instead of a slam::Set to instantiate the ProductSet.

This change will be made within Multimat itself in #945, but for now we just construct the fully-specified BivariateMap ourselves to have a performance "target" to aim for.

We also standardize on the use of BivariateMap::operator()(firstIdx, secondIdx) for direct-access, since what we really care about is the performance hit from the abstraction.