use Dataset.sizes instead of Dataset.dims for getting dimension sizes

[niksirbi]

Description

What is this PR

• [x] Bug fix
• [ ] Addition of a new feature
• [ ] Other

Why is this PR needed?

Multiple instances of the following warning are being raised during tests:

FutureWarning: The return type of Dataset.dims will be changed to return a set of dimension names in future, in order to be more consistent with DataArray.dims. To access a mapping from dimension names to lengths, please use Dataset.sizes

What does this PR do?

It uses Datasets.sizes instead of Dataset.dims when getting the lengths of each dimension. Dataset.dims is still in use when we simply want the names of the dimensions.

How has this PR been tested?

Existing tests still pass.

Is this a breaking change?

No.

Does this PR require an update to the documentation?

No

Checklist:

• [x] The code has been tested locally
• [ ] Tests have been added to cover all new functionality
• [ ] The documentation has been updated to reflect any changes
• [x] The code has been formatted with pre-commit

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.28%. Comparing base (736eaaa) to head (74d62d0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #153 +/- ## ======================================= Coverage 99.28% 99.28% ======================================= Files 9 9 Lines 557 557 ======================================= Hits 553 553 Misses 4 4 ```

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