:bug: VaryRun not varying all iteration variables

[clmould]

Describe the bug

When running the VaryRun example (#4048), all the iteration variable values are changed except for the value of f_nd_impurity_electrons(13) , this is potentially a bug?

Steps to reproduce

Run the VaryRun example and look at the table of changes produced at the end - all the values have changed except the value of f_nd_impurity_electrons(13)

Expected behaviour

All the iteration variable values should've changed

Evidence

All the iteration variable values should change?

Environment

• OS:
• Version:

Additional context