Rename coordinate systems in visium

[grst]

Hi @LucaMarconato,

here I propose to rename the coordinate systems to the dataset id as discussed on discourse. Having unique names makes it easier when working with multiple samples.

With this change reading in a visium sample with dataset_id sample2 results in the following spatialdata object:

SpatialData object
├── Images
│     ├── 'sample2_hires_image': DataArray[cyx] (3, 2000, 2000)
│     └── 'sample2_lowres_image': DataArray[cyx] (3, 600, 600)
├── Shapes
│     └── 'sample2': GeoDataFrame shape: (523, 2) (2D shapes)
└── Tables
      └── 'table': AnnData (523, 18085)
with coordinate systems:
    ▸ 'sample2', with elements:
        sample2_hires_image (Images), sample2_lowres_image (Images), sample2 (Shapes)
    ▸ 'sample2_downscaled_hires', with elements:
        sample2_hires_image (Images), sample2 (Shapes)
    ▸ 'sample2_downscaled_lowres', with elements:
        sample2_lowres_image (Images), sample2 (Shapes)

If you agree with this change, I can try to also apply it to the other readers.