refactor: Move functional reduction to fields.

[prmukherj]

Context

Since we are refactoring the PyFluent repo, it might be a good idea to bring out the functional reduction (with multiple session context) from solver and put it under fields.

Change Summary

fields/reduction/
init.py
abstract_reduction.py
reduction.py # Existing session-bound Reduction
functional.py # Existing functional API

Rationale

As both operate on Fluent field quantities, thus it makes this a reasonable home for it.

Impact

Users to see better grouping and better API reference generated.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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