Add feature to aggregate timeseries at sub-annual time resolution

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Description of PR

This PR implements a first version of an aggregate_time() feature: it applies a function (default sum) data given at a sub-annual time resolution to a yearly or more aggregated sub-annual time slide. The PR adds a new tutorial for this feature, to be extended in future PRs.

Examples:

df.aggregate_time(<variable>)  # to get the sum of `variable` over the entire year
df.aggregate_time(<variable>, value='summer', components=['june', 'july', 'august'],
    method='max')  # to get the maximum of `variable` over the summer months

[coveralls]

Coverage increased (+0.08%) to 86.37% when pulling 082078a on feature/aggregate_time into 8e1393e on master.

[danielhuppmann]

rebased following release v0.5.0

[gidden]

Updated release notes, will merge when CI passed. Thanks a bunch here @danielhuppmann !