Simplify mapping arguments

[lazappi]

Simplify the mapping arguments in conversion functions to use named vectors instead of nested lists.

Related to (replaces?) #251

• All mapping arguments now expect named vectors instead of lists
• Complex mapping from other slots etc. is now not possible. Users will need to make these changes manually if required.
• The exception is the reduction_mapping argument of from_Seurat()/from_SingleCellExperiment() which allows a more complex list format if users want to specify loadings etc.
• Simple named lists will still work but are not shown in examples
• Unnamed vectors will also work, c("item1", "item2") gets converted to c(item1 = "item1", item2 = "item2"). This is mostly to avoid simple errors and isn't shown in examples.

This helps standardise things so it will be simpler to replace from_Seurat()/from_SingleCellExperiment() with as_AnnData(). More updates to documentation will come in #253.