Change output variable names and vastly improve settings checks

[aaronjridley]

Description

Addresses #70 and #144

The output variable names are changed to not have spaces or parentheses.

The input settings checking is simplified and made more consistent - now developers can simply use the get_settings_xx(key, key) functions. These functions check if the keys exist and then set an error flag if they don't. Much more robust!

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change that adds functionality)

How Has This Been Tested?

Run the default test case.

One issue with testing these things is that the default.json file has correct settings, so if the user sets a setting incorrectly in the aether.json file, it will simply ADD to the settings json and not overwrite the default setting, since this is how json merge works.

Test configuration

• Operating system: WSL Ubuntu
• Compiler, version number: gcc 11

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