[WIP] Addresses issue #65, configuration variable names denote sense.

The configuration variables are changed as follows:

- CA1elevation -> CA1extension
- CB1elevation -> CB1extension
- CA1abduction -> CA1adduction
- A1A2flexion -> A1A2extension
- B1B2flexion -> B1B2extension
- TClateralSpinalFlexion -> TCsagittalSpinalFlexion
- PJ1flexion -> PJ1extension
- PK1flexion -> PK1extension
- PJ1abduction -> PJ1adduction

Tasks:

- [x] Update configuration figure with new variable names.
- [x] Implement deprecation and deprecated warnings.
- [x] Replace usages of old configuration names with the new configuration names.
- [x] Write tests to ensure deprecated variable names still work.
- [x] Update all other tests to use the new variable names.
- [x] Ensure the GUI uses the updated names, and that the new names correctly denote sense.
- [x] Update all configuration.txt YAML files (including test YAML files) to use the new variable names.