[newchem-cpp] internal scratch-buffer interface update

[mabruzzo]

This PR was originally proposed as brittonsmith#26

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This must be reviewed after #332.

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This consists of a few commits that tweak the general interface of the internal-type I have been creating (their nominal role is to carry around collections of scratch buffers). Basically I formalize the idea that each of these types should support the visitor design pattern for visiting the data-members of these types (we were already doing to help with allocation and destruction).
A longer description of the design pattern is provided within the modified files.

This is important for the at least the initial transcription of step_rate_newton_raphson. We may be able to get rid of this machinery down the road.

While the new visitor machinery makes fairly heavy use of C++, it's relatively compartmentalized. In my view this makes the more C-like nature of the rest of the code a lot more ergonomic. But, we can always revisit this later.

This machinery could be very useful for aggregating memory allocations, the initial port to GPUs (i.e. copying data to and from the GPU), and debugging (e.g. printing out the contents of the buffers tracked by a particular buffer in a nicely formatted manner)

[brittonsmith]

@mabruzzo can you resolve these merger conflicts?