Silmathoron/NNGT: Version 1.3.0: updated backends, groups and improved documentation

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. 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 algorithm