position parameter bug for  RunMCA.Seurat()

https://github.com/RausellLab/CelliD/blob/e306b43e05a7d895d6d7fa7b149c9091467fe71c/R/mca.R#L77

Here,should add the 
```
 data_matrix <- as.matrix(GetAssayData(X, slot=slot)) 
```
or it will return bug with
```
r$> data_matrix <- as.matrix(GetAssayData(brain, "data"))
Error in `GetAssayData()`:
! `assay` must be one of "Spatial" or "SCT", not "data".
```