More flexible colnames of `imgData`

[senbaikang]

Hi,

Currently, the sanity check of imgData requires the DataFrame object to have exactly four columns c("sample_id", "image_id", "data", "scaleFactor"). Do you consider making it more flexible by checking instead their existance?

I am asking this because I have a use case where the spot diameter is of the interest. I know this is only for Visium, but it would be a lot more convenient for me if it goes into imgData.

#143

Thank you!