[TASK] Allow pass-through variables to custom functions from the input file

[dylanjm]

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Issue Description

Is your feature request related to a problem? Please describe.

Some users have expressed the desire to be able to pass certain variables from input files into the custom function declared in a separate .py file.

This issue may require some discussion on the best way to implement it.

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For Change Control Board: Issue Review

This review should occur before any development is performed as a response to this issue.

• [x] 1. Is it tagged with a type: defect or task?
• [x] 2. Is it tagged with a priority: critical, normal or minor?
• [x] 3. If it will impact requirements or requirements tests, is it tagged with requirements?
• [x] 4. If it is a defect, can it cause wrong results for users? If so an email needs to be sent to the users.
• [x] 5. Is a rationale provided? (Such as explaining why the improvement is needed or why current code is wrong.)

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For Change Control Board: Issue Closure

This review should occur when the issue is imminently going to be closed.

• [x] 1. If the issue is a defect, is the defect fixed?
• [x] 2. If the issue is a defect, is the defect tested for in the regression test system? (If not explain why not.)
• [x] 3. If the issue can impact users, has an email to the users group been written (the email should specify if the defect impacts stable or master)?
• [x] 4. If the issue is a defect, does it impact the latest release branch? If yes, is there any issue tagged with release (create if needed)?
• [x] 5. If the issue is being closed without a pull request, has an explanation of why it is being closed been provided?