update seurat5 count matrix access

[chilampoon]

updated seurat5 count matrix access. After some testing, it's working well with the basic examples from the vignette.

[rrydbirk]

@evanbiederstedt @kdragicevic - looks fine to me. Merge?

[VPetukhov]

Thank you for the update!

The old code uses a lot of @, which relies on specific Seurat versions and isn't recommended according to Seurat documentation. I wasn't aware of that when I wrote it, but now to make the code more stable we should avoid using @. I suggest rewriting data access as object[['RNA']]$counts.