Add mean aggregation function

[Aritra8438]

Fixes #2915

This pull request adds support for Mean aggregation as a transformation step in the data explorer.

User will be able to:

• Get Mean of integer columns based on Group By clause.
• Get Mean of decimal columns based on Group By clause.

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Technical details

I have made the following changes:

• Add Mean(DBFunction) class to support mean aggregation
• Add mean suffix to the output alias of the aggregated column.
• Add Mean to the list of transformation steps in the data explorer.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[mathemancer]

Back end changes look good. To merge, you need to:

• sort out merge conflicts.
• get the front end bit added.

[Aritra8438]

Hi @mathemancer and @seancolsen. This PR is stacked on #2914, which is again stacked on #2912.

So, the ideal order of merging will be, #2912 -> #2914 -> #2916.

[mathemancer]

Nice work as usual!