Updates AVG to return DECIMAL

[johnedquinn]

Relevant Issues

• None

Description

• Updates AVG to return DECIMAL. Increases conformance by 4 tests.

Other Information

• Updated Unreleased Section in CHANGELOG: NO
• Any backward-incompatible changes? YES -- updates AVG to return DECIMAL
• Any new external dependencies? NO
• Do your changes comply with the Contributing Guidelines and Code Style Guidelines? YES

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.