Add version checking for dictionary calls

[exowanderer]

Main Issue

Using the DeBaCl example Jupyter notebok with Python3 failed at several places -- all but one of which dealt with dictionary calls such as itervalues(), iteritems(), iterkeys() -- because the syntax for dictionary calls changed between Python2 and Python3.

There were 12 such cases in the file level_set_tree.py.

After adding code snippets (listed below) that checked for python2 vs python3, I was able to fully run the example/getting_started.ipynb to completion with reasonable results (vs expectations).

A second issue was found while investigating any python3 versioning dictionary call issues. The line n_bigkid = sum(_np.array(kid_size.values()) >= threshold) was changed to

n_bigkid = sum([len(tree.nodes[k].members) for k in parent.children]) (line 665 in this PR code)

The above error occurred because python (maybe only python3) cannot compare dictionary values with integers: "dict_values >= int".

Using the one line for-loop inside a summation above fixed the issue. Please confirm that the two lines fulfill the same function.

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Example Snippets

This PR adds an import at the top of the package: from sys import version and 24 if-statements (12 places x 2 versions) to check the version. Such as:

if version[0] == '2':
    nodes_items = self.nodes.iteritems()
if version[0] == '3':
    nodes_items = self.nodes.items()

if version[0] == '2':
    nodes_values = self.nodes.itervalues()
if version[0] == '3':
    nodes_values = self.nodes.values()

if version[0] == '2':
    root_tree_nodes_keys = root_tree.nodes.nodes()
if version[0] == '3':
    root_tree_nodes_keys = root_tree.nodes.keys()

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Necessary Version Information:

Darwin local 15.6.0 Darwin Kernel Version 15.6.0: Mon Aug 29 20:21:34 PDT 2016; root:xnu-3248.60.11~1/RELEASE_X86_64 x86_64

Python 3.5.2 |Anaconda custom (x86_64)| (default, Jul 2 2016, 17:52:12) \n[GCC 4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.28)]

[bpkent]

Hi @exowanderer, many thanks for the PR - I'm currently unable to work on DeBaCl, but I'm hoping to return to it soon.
-Brian