Handling list of DICOM datasets reference when creating rtstruct

[gacou54]

We plan to use rt-utils in our pipeline to create RTStruct. Our problem is that the reference DICOM data need to be on the filesystem. For situations where we only manipulate data in memory, I think create_new() and create_from() should be able to take a List[pydicom.Dataset].

series_datasets: List[pydicom.Dataset]
rtstruct = RTStructBuilder.create_new(series_datasets)

I modified the signature of these two functions and added/adapted the tests.

I hope the PR is OK and "fits" in rt-utils. Otherwise, do not hesitate to suggest changes.