Add arg for sce methods to name reducedDim output

[Robert Amezquita]

Sometimes need to generate multiple PCAs based on different underlying assay data. Would be great to add an arg for the SCE method

runPCA(sce, ..., name = "my_pca")

[Aaron Lun]

Goes in scater.

[Robert Amezquita]

Thanks! Moving over to scater, closing here.