Data mapping not fully reflected in exports

[axelboc]

Describe the bug

The export feature (to CSV, TIFF, etc.) does not take into account potential transposition of datasets (or dataset slices). See https://github.com/silx-kit/vscode-h5web/pull/23#discussion_r1257530624

The getExportURL method that data providers may implement does not receive sufficient information to determine whether users have transposed the dataset/slice they want to export. The current signature of the method is:

public getExportURL?<D extends Dataset<ArrayShape>>(
    format: ExportFormat,
    dataset: D,
    selection: string | undefined,
    value: Value<D>
  ): ExportURL;

To Reproduce

1. Go to https://h5web.panosc.eu/h5grove
2. Select dataset /entry_0000/1_integration/results/I
3. Select Matrix visualization
4. Export to CSV => 10 rows x 1000 columns
5. Use dimension mapper sidebar to transpose dataset
6. Export to CSV again => still 10 rows x 1000 columns (expected 1000 rows x 10 columns)

Can also be reproduced with the Heatmap visualization and the TIFF export, for instance.

Expected behaviour

Exports should reflect potential transposition of the datasets (or dataset slices).