More consolidation of Hashtable derived types.

[ctb]

Briefly, this PR:

• interposes a new Hashgraph CPython class that inherits from Hashtable and becomes a new base for Countgraph and Nodegraph (in the CPython interface).
• articulates the C++ internals so that they use Hashtable and Hashgraph appropriately (as well as Countgraph etc. where absolutely necessary).
• renames the C++ and CPython API to match: Counttable, Nodetable, Countgraph, Nodegraph.

These changes make a clear distinction between 'tables' and 'graphs' - tables have all of
the basic functionality needed for counting, while graphs support various traversal methods.
This paves the way for:

(a) adding new hash functions, including irreversible ones supporting k > 32 for the *table objects; and
(b) building out a new Counttable CPython object that will support the non-graph operations for k > 32.

(I don't like the 'Counttable' name that much but it fits with Countgraph. I suppose we could do Countstable. But that might engender confusion. Thoughts?)

• Is it mergeable?
• make test Did it pass the tests?
• make clean diff-cover If it introduces new functionality in
  scripts/ is it tested?
• make format diff_pylint_report cppcheck doc pydocstyle Is it well
  formatted?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Is it documented in the ChangeLog?
  http://en.wikipedia.org/wiki/Changelog#Format
• Was a spellchecker run on the source code and documentation after
  changes were made?
• Do the changes respect streaming IO? (Are they
  tested for streaming IO?)
• Is the Copyright year up to date?

[codecov-io]

Current coverage is 95.76% (diff: 100%)

Merging #1504 into master will increase coverage by 0.02%

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  Lines          2938       2952    +14   
  Methods           0          0          
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  Branches        449        449          
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+ Hits           2813       2827    +14   
  Misses           55         55          
  Partials         70         70          

Powered by Codecov. Last update e2d1132...6303cb8

[ctb]

Ready for initial review, y'all. @betatim @luizirber @camillescott @standage. Might be easier to go commit by commit :(.

Still have to check out my modifications to the abundance dist functions, and I'm not sure if I should add tests for Counttable and Nodetable on this PR, but I think most of it is done.

[betatim]

Prefer Counttable over Countstable. More substantial comments later

[ctb]

• look into using our saner inheritance hierarchy to simplify hashtable extraction in the CPython code.

[betatim]

Do we need this here if it is just empty?

[ctb]

I went both ways on this - it's nice to have it there for if/when we add methods... but yeah, I guess no need.

[betatim]

Delete all the following lines

[betatim]

delete me

[betatim]

Should we keep this around or delete?

[camillescott]

I'd like to see these bits of code detritus go, but I think perhaps that should get its own PR -- there's a fair amount of it.

[betatim]

If you move this up to be the first definition in the method you (probably) don't have to pay for a copy when returning it.

[ctb]

OK, but I don't understand why :)

[betatim]

https://en.wikipedia.org/wiki/Return_value_optimization it probably isn't that exciting in this case but for other/bigger/expensive to copy types it might be more interesting

[betatim]

I assume that the substantial bits of code that were moved didn't get modified when you moved them because the tests still pass.

[betatim]

Do you know who owns the reference to the list that comes from PyArg_ParseTuple?

[betatim]

PyArg_ParseTuple doesn't increase the reference count, and presumably the ref count can't decrease to zero while we are using an argument to this function.

[ctb]

Yep (or at least not with the GIL held).

[ctb]

Let's ahead and push the magic merge button when the build ends! (but please don't squash commits :)

[camillescott]

Doh, I was too late :(

[ctb]

wassup @camillescott?