Create Network semantic type

[BenKaehler]

Dependencies: #1

Prerequisites:

1. Python programming.
2. Understanding of QIIME 2 semantic types and how to create new ones.

Create a Network QIIME 2 semantic type. It should provide transformers for reading and writing the igraph data structures saved be the R script.

Outdated example with lots of explanatory comments: https://github.com/qiime2-graveyard/q2-dummy-types/tree/master/q2_dummy_types.

Real-world working example: https://github.com/qiime2/q2-dada2/blob/master/q2_dada2/_stats.py and https://github.com/qiime2/q2-dada2/blob/master/q2_dada2/_transformer.py. Background: https://dev.qiime2.org/latest/storing-data/.