Decoupler: allow multiple merges on AnnData

[pcm32]

Description

This adds functionality for the decoupler pseudo_bulk to be able to merge multiple groups of fields and not just a single one.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• I have made any required changes to upstream dependencies for a tool wrapper, and they are available in distribution channels (e.g. Pip, Conda).
• If I have updated the underlying software for a tool wrapper (e.g. scanpy-scripts by changing the value of @TOOL_VERSION@), then I have reset all 'build' values to 0 (e.g. @TOOL_VERSION@+galaxy0)
• If I have updated a tool wrapper without a software change, then I have bumped the associated 'build' values (e.g. @TOOL_VERSION@+galaxy0 @TOOL_VERSION@+galaxy1). It is acceptable to do this as well when the cli version changed but not the underlying tool (to avoid issues in the coming point).
• If I changed the version, the @TOOL_VERSION@ part of the version does not contain any + symbols within, otherwise this will break tool ordering on the interface and the default tool being picked. Tool version should always conform to PEP440 to avoid this issue. The only + should be the one preceding galaxy<build> (unless that all the versions from that tool previously followed a different pattern).