[WIP] Sphinx documentation / mass renaming

[tristan0x]

Mass Refactoring

• use graph lexical field, for instance:
  • s/node/vertex
  • s/connection/edge

TravisCI:

• reformat YAML
• fix: add missing HDF5 dependency
• fix: always use Python 3

CLI:

• new "doc" command to spawn a web server hosting the package documentation

Python API:

• New API to describe graph topology
• new module basalt.ngv.graph based on new API
• Pybind11 bindings are more Pythonic

Documentation:

• new sphinx documentation for both Python package and C++ library

C++ library:

• use #pragma once
• compile with GCC -Wshadow warning
• documentation public API with Doxygen
• split Python bindings in multiple files