Nngt

This patch fixes an issue affecting the "nngt" backend (for users not using any of the standard graph libraries) that led to weights set after edge creation (using ``set_weights``) to be incorrectly updated in some cases, notably when using NNGT with NEST.

(note that the Travis test for graph-tool on Python 2 fails solely because Tiago is no longer supporting it and removed the package from his repo, it is not linked to any issue with this release)

This patch fixes a bug using the ``fixed_degree`` or ``gaussian_degree`` methods on different sources and target neurons with one of the following functions:
* ``connect_groups``, ``connect_neural_groups``
* ``connect_neural_types``
* ``connect_nodes``

It also deprecates ``connect_neural_groups`` in favour of ``connect_groups``.

**NB:** this release will be the last one supporting python 2 and NEST 2, starting NNGT 2.0, only python 3 and NEST 3 will be supported.
**EDIT:** due to delays in the release of NEST3, support for NEST2 was maintained in NNGT 2.

**Improvements:**
* support for networkx 2.4
* added ``MetaGroup`` class
* added excitatory/inhibitory access for ``NeuralPop`` and ``MetaGroup``

**Bugfixes:**
* corrected betweenness for all backends
* corrected multiple node access for degrees in "nngt" backend
* corrected plot import for ``simulation`` module

**Deprecations:**
* sunsetting Python 2
* deprecated ``nest_gid`` in favor of ``nest_gids``

**Under the hood:**
* Improved testing and coverage
* switched to sets in generation algorithms

**Breaking change:** `decimate` converted to `decimate_connections` in plot functions.

**Various corrections:**

* improved setup for NEST loading
* set binsize to int in `total_firing_rate`
* improved data management and pip install correction

This new version vastly improves the support of the backends with respect to node and edges attributes.
It also introduces new metagroup objects for even better network generation.
Finally, it comes with a refreshed and enriched documentation.

**Breaking changes:**

* ``ntype`` was switched to ``neuron_type`` for ``NeuralGroup`` and ``NeuralPop``
* ``create_group`` takes ``neurons`` as first parameters and ``name`` as second

**Improvements:**

* full attributes support (including strings and objects) for all backends
* addition of metagroups
* new ``get_nodes`` and ``get_edges`` functions
* improved plotting and histogram readded
* enriched and updated documentation and tutorials
* larger coverage (now at 30%)

**Bugfix:**

* corrected multiple issues with networkx and nngt backends
* fixed all attribute-related issues
* corrected IO for ``Network`` with ``NeuralPop``

This new version provides a working multithreading install on Windows, as well as several improvements and bugfixes.

**Breaking change:** for interactions with NEST, the ``set_minis`` has been change to have independent weight for the noise.

**Improvements:**

* Improved and corrected draw_network, plot_activity, and animation
* IO now save and load properly the areas of a Shape

**Bugfix:**

* Corrected error in distance-rule with OpenMP
* Corrected error in betweenness with igraph
* Corrected neighbors with networkx
* Fixed crash with sorting method

**Notes:**

* I realized that GitHub does not include the submodules in the release archives; starting now, I will also include the .tar.gz and .zip archives containing the same files as the corresponding version of NNGT on PyPi.
* For installation, it is now recommended to install directly through ``pip``.

To install via ``pip``, use either

pip install --user nngt

or

sudo pip install nngt