Make code that retrieves values from obs/X/layer/raw of the different modalities a public function

[grst]

Is your feature request related to a problem? Please describe.
In scirpy, I have a custom extension of "pl.embedding" that plots clonotype networks. Similar to the pl.embedding functions in muon and scanpy, nodes can be colored by either variables from obs, or by genes stored in X/raw/layers.

Muon provides a mechanism (which is currently somewhat broken, see #96) to retrieve values from either of the slots by specifying keys such as "gex:CD8A", "gex:sample", "patient" together with the parameters use_raw, and layer.

To achieve the same, I duplicated a lot of code from mu.pl.embedding into scirpy.

To avoid this, it would be helpful to have this as a public function in muon, e.g.

def get(
    mdata: MuData,
    variable: str,
    *,
    use_raw: bool = False,
    layer: Optional[str] = None
) -> pd.Series

Alternatives
Maybe this should even be a method in MuData?

MuData.get("gex:CD8A")

Additional context
With scirpy v0.13, scirpy introduces support for MuData. Previously everything was in a single AnnData object, therefore this wasn't a problem before.