Give warning when overwriting tables during normalization

[0xMuluh]

Currently transformAssay() does not issue warning when overwriting tables.
Issue was first identified here: hanhineva-lab/notame#10

[antagomir]

Hmm true but incomplete - it does not issue warning. Insted it fails, which is stronger.

I would suggest that the failure is changed into a warning.

However the same policy would be then ideally any method that returns named assays.

library(mia)
data(peerj13075)
tse <- peerj13075
tse <- transformAssay(tse, method="relabundance", assay.type="counts", name="counts")

Error: 'name' must be a non-empty single character value and be different from assay.type.