[BUG] Message Exchange

[oscarhua]

Description

Addresses #135

Now in the cubesphere grid, ghost cells are aligned with the current face and their values are linearly interpolated from other faces

Type of change

• Bug fix (non-breaking change that fixes an issue)

How Has This Been Tested?

Run cmake with -DTEST_EXCHANGE:BOOL=TRUE and required changes to aether.json are described in src/main/main_test_exchange.cpp

Test configuration

• Operating system: Ubuntu-20.04
• Compiler, version number: gcc 9.4.0

Checklist:

• [N/A] Make sure you are merging into the develop (not master) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• [N/A] Add a note to CHANGELOG.md, summarizing the changes