Changes proposed in this Pull Request

• Clarify issue in Minor improvement of snapshots handling #1091
• Remove n._investment_periods, since its data is included in n.snapshots and therefore not needed.
• New network attributes n.timesteps, n.periods (alias for n.investment_periods) and n.has_periods.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.