Refine network representation

[FabianHofmann]

Addresses #333

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• A note for the release notes doc/release_notes.rst of the upcoming release is included.

[FabianHofmann]

For non-empty networks:

PyPSA Network 'AC-DC'
Components:
 - Bus: 9
 - Carrier: 3
 - Generator: 6
 - GlobalConstraint: 1
 - Line: 7
 - Link: 4
 - Load: 6
Snapshots: 10

for empty networks:

Empty PyPSA Network
Components: none
Snapshots: 1