Add example for vector data cube

[danlooo]

I found it a bit difficult to use mapCube in situations, where dimensions are both removed and created. Thus, I added an example on how to create a vector data cube that aggregates points of the same region. This example is different from the others mentioned in file create_from_func.jl, because the transformation is not calculated elementwise as it is one in f(lo, la, t) = (lo + la + Dates.dayofyear(t)) but requires information from the other dimensions as well, i.e. we need the longitude and latitude to calculate the aggregated value.

[coveralls]

Pull Request Test Coverage Report for Build 5398976456

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 75.307%

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Totals
Change from base Build 5376433093:	0.0%
Covered Lines:	1348
Relevant Lines:	1790

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💛 - Coveralls

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (1d44e97) 75.34% compared to head (c30d8fd) 75.34%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #270 +/- ## ======================================= Coverage 75.34% 75.34% ======================================= Files 12 12 Lines 1858 1858 ======================================= Hits 1400 1400 Misses 458 458 ```

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