New module: anndata/getsize module

[LeonHafner]

Adding the module anndata/getsize which outputs the size of an anndata object.
This can then be used to add the size of the anndata object to its meta map:
ANNDATA_GETSIZE.out.size.map{meta, txt -> [meta, txt.text.toInteger()]}

Currently there is no proper H5AD data on the test-datasets modules branch. For now I used the files from the scdownstream pipeline, but I will try to add proper test data to the modules asap.

PR checklist

Closes #6923

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
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[SPPearce]

Generally fine.
It would be good to put a h5ad file into the modules test-datasets though, are you able to do that?

[SPPearce]

Looks good to me now, thanks for putting the file into the modules testdata repo.