Add arguments to select what is converted

[lazappi]

From my own work sometimes it would be nice to be able to control what is converted, mostly to avoid duplicated things that aren't needed or to avoid converting something that fails (although checks should hopefully avoid that).

Arguments should let users select for each slot if all of it is converted, none of it or some selection. For example for assays in SCE2AnnData():

• assays = TRUE - All assays are converted (default)
• assays = FALSE - No assays are converted
• assays = c("counts") - The counts assay would be converted but not any others (logcounts etc.)

[lazappi]

@LTLA Any objections to adding this functionality (or reasons it's a bad idea)?

[LTLA]

Sounds reasonable to me.

[lazappi]

Done in devel