Clarify return float/int type for core.Composition.reduced_composition and siblings, minor type clean up for core.Composition

[DanielYang59]

Summary

• Clarify return float/int type for core.Composition.reduced_composition and siblings, factor returned by reduce_formula would be int instead of float (as a union type of float | int):

  pymatgen/src/pymatgen/core/composition.py

  Lines 1345 to 1348 in 296a0ec

  	factor = 1
  	# Enforce integers for doing gcd.
  	if all(int(i) == i for i in sym_amt.values()):
  	factor = abs(gcd(*(int(i) for i in sym_amt.values())))

• Minor type clean up for core.Composition
• reduce_formula is not covered by test