Use gms::getLine() instead of re-defining it everywhere

[mikapfl]

Purpose of this PR

Instead of defining getLine or variants of it in every script, use the version from gms. This is not a new dependency, we depend on gms anyway in REMIND. See discussion in #929.

Type of change

• Refactoring

Checklist:

• My code follows the coding etiquette
• I have performed a self-review of my own code
• Changes are commented, particularly in hard-to-understand areas
• I have updated the in-code documentation
• I have adjusted reporting where it was needed
• The model compiles and runs successfully (Rscript start.R -q)