docs: basic stuff about the module structure

As a bonus, in the Python shell, `help(gnpy)`, `help(gnpy.core)`, etc,
now produce at least some useful information.

Change-Id: I76ade6f2456fcebd3c0a147374815dd245dc4b10

gnpy/__init__.py

@@ -0,0 +1,8 @@
+'''
+GNPy is an open-source, community-developed library for building route planning and optimization tools in real-world mesh optical networks. It is based on the Gaussian Noise Model.
+
+Signal propagation is implemented in :py:mod:`.core`.
+Path finding and spectrum assignment is in :py:mod:`.topology`.
+Various tools and auxiliary code, including the JSON I/O handling, is in
+:py:mod:`.tools`.
+'''

gnpy/core/__init__.py

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+'''
+Simulation of signal propagation in the DWDM network
+
+Optical signals, as defined via :class:`.info.SpectralInformation`, enter
+:py:mod:`.elements` which compute how these signals are affected as they travel
+through the :py:mod:`.network`.
+The simulation is controlled via :py:mod:`.parameters` and implemented mainly
+via :py:mod:`.science_utils`.
+'''

gnpy/tools/__init__.py

@@ -0,0 +1,5 @@
+'''
+Processing of data via :py:mod:`.json_io`.
+Utilities for Excel conversion in :py:mod:`.convert` and :py:mod:`.service_sheet`.
+Example code in :py:mod:`.cli_examples` and :py:mod:`.plots`.
+'''

gnpy/topology/__init__.py

@@ -0,0 +1,3 @@
+'''
+Tracking :py:mod:`.request` for spectrum and their :py:mod:`.spectrum_assignment`.
+'''