Add SingleCellExperiment support

[czackl]

Since omnideconv can import SingleCellExperiments we should enable users to upload those in the app

[federicomarini]

An easy way - if the redDim slots and co are not really used: We can simply "cast" that to be a simple SummarizedExperiment object.
Which in the end, it already is, but has some extra slots that one probably won't need here.

That being said: a SCE is already an SE, so ideally we probably don't even need to do anything - what will happen is that simply some of the information in it just won't be used.

Heads up! The check should not be done using the class() function, because that does not capture the fact that SCE is also an SE.
See this chunk to reproduce the idea of it:

> library(SingleCellExperiment)
> 
> my_sce <- SingleCellExperiment::SingleCellExperiment(
+     assays = List(
+         logcounts = matrix(rnorm(1000, mean = 3, sd = 1), nrow = 100)
+     ),
+     reducedDims = List(
+         PCA = matrix(rnorm(20), nrow = 10)
+     )
+ )
> 
> my_se <- SummarizedExperiment::SummarizedExperiment(
+     assays = List(
+         logcounts = matrix(rnorm(1000, mean = 3, sd = 1), nrow = 100)
+     )
+ )
> 
> class(my_sce)
[1] "SingleCellExperiment"
attr(,"package")
[1] "SingleCellExperiment"
> class(my_se)
[1] "SummarizedExperiment"
attr(,"package")
[1] "SummarizedExperiment"
> 
> ## but 
> 
> is(my_sce, "SingleCellExperiment")
[1] TRUE
> is(my_sce, "SummarizedExperiment")
[1] TRUE
> is(my_se, "SingleCellExperiment")
[1] FALSE
> is(my_sce, "SummarizedExperiment")
[1] TRUE

[federicomarini]

... with is(my_sce, "SummarizedExperiment") being the key of the whole "shortcut"