New approximately EC compressible Euler flux

[Comprehensive analysis of entropy conservation property of non-dissipative schemes for compressible flows: KEEP scheme redefined](https://doi.org/10.1016/j.jcp.2022.111494)

CC @jlchan

The issue provides only a link to a paper about entropy conservation and a request for a new approximately EC compressible Euler flux; it names no files, tests, or entry points. Start by reading the linked paper and locating the flux implementation and related tests in Trixi.jl, then define the required behavior and validation before deciding what completion entails.

Written by the indexing model from the issue text.