Implement alternative resampling methods

[BSchilperoort]

Users may want to use different statistics rather than "mean" for their resampling. This PR adds that functionality. All of the following (numpy) statistics are supported:

• mean, min, max, median
• std, var, ptp (peak-to-peak)
• nanmean, nanmedian, nanstd, nanvar

Additionally, user defined functions can also be passed.

Tasks:

• [x] Update xarray code
• [x] Update pandas code
• [x] Add tests
• [x] Fix #29
• [x] Explain in notebook (depends on #26)

Note: the traintest tests had to be updated due to the change in the sorting of the anchor years in the xarray output of resample.

Closes #4

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[BSchilperoort]

Note that the md link check fails. This is fixed in #28, so I won't fix the link here.

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

93.1% Coverage
0.0% Duplication

[geek-yang]

The custom function looks good and works well too. Let's merge this PR!

[BSchilperoort]

Thanks for the reviews @geek-yang !