Feat: Propagate global properties into variants

[GMinucci]

What does this PR do

Propagates global custom properties to all variants

How can it be tested

Add a global custom property and confirm it's available in the variant generated custom properties

Task

resolves #247
resolves #243

Checklist:

• I ran make validation locally with success
• I have not introduced new bugs
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new errors