Compatibility of ep.pp.explicit_impute with different datatypes

[eroell]

Description of feature

Follow the template workflow of #835:

• introduce singledispatch to check for allowed data types, or raise an error if others are encountered
• adapt the tests to check these data types

Create underscore private function for each datatype, and keep the adata object as primary

[aGuyLearning]

A problem arrises when testing a dask array. the impute method works, it just fails during the quality control step of _warn_imputation_threshold. the qc_metrics function calls _var_qc_metrics function, where it fails on an indexing issue. Could also be an issue with #851, once more datatypes should be supported