Change how cellassign celltypes are saved

[sjspielman]

In #402, we saved CellAssign output in the SCE object.
Turns out that that content saved into the colData celltype column was actually a full data frame and not just a vector celltypes, eg.

> sce$cellassign_celltype_annotation |>head()
           barcode            celltype prediction
1 ACGTCAAAGGTGTTAA           Monocytes  1.0000000
2 CCTTCGAGTGTCTGAT               other  0.9999851
3 GCGGGTTAGCCCTAAT Gamma delta T cells  1.0000000
4 AGCGTATAGACACTAA Gamma delta T cells  1.0000000
5 GCAAACTCATATGGTC Gamma delta T cells  1.0000000
6 TACGGTACATTATCTC           Monocytes  1.0000000

We will want to just save celltype here, so this line of code needs to change:

scpca-nf/bin/classify_cellassign.R

Lines 71 to 72 in 78eb330

	# add cell type and prediction to colData
	sce$cellassign_celltype_annotation <- celltype_assignments

to:

sce$cellassign_celltype_annotation <- celltype_assignments$celltype

That's it!

[sjspielman]

Closed with #421