Warn that matrix dimnames are not stored in `uns`

[LouiseDck]

h5ad_file <- system.file("extdata", "example.h5ad", package = "anndataR")
adata <- read_h5ad(h5ad_file)

# generate random matrix with dimnames
named_matrix <- matrix(rnorm(20), nrow = 5, ncol = 4)
rownames(named_matrix) <- paste0("Row", 1:5)
colnames(named_matrix) <- paste0("Col", 1:4)

adata$uns[["named_matrix"]] <- named_matrix
#             Col1       Col2       Col3        Col4
# Row1  0.44347140 -0.3506371  1.0323446 -0.03400620
# Row2 -0.99902193  0.1735424 -0.2974675  0.39795719
# Row3  1.15786673  0.5032449 -0.1217729 -0.05625188
# Row4  0.08085226  1.8353912 -1.4228116 -0.38971408
# Row5  0.14804978  1.4974531 -0.3820515 -0.16296954

adata$write_h5ad("output.h5ad")

adata2 <- read_h5ad("output.h5ad")
adata2$uns[["named_matrix"]]
#             [,1]       [,2]       [,3]        [,4]
# [1,]  0.44347140 -0.3506371  1.0323446 -0.03400620
# [2,] -0.99902193  0.1735424 -0.2974675  0.39795719
# [3,]  1.15786673  0.5032449 -0.1217729 -0.05625188
# [4,]  0.08085226  1.8353912 -1.4228116 -0.38971408
# [5,]  0.14804978  1.4974531 -0.3820515 -0.16296954

We should at least issue a warning when writing.

[lazappi]

We should check what Python does and match that. Possibly there should also be warnings for other slots (i.e. a matrix in obsm). It would make sense to do this when things are added rather than written to disk so it might fit in #328/#324. Also related to #327.