Update parameter sweep tool to support indexed vars

[andrewlee94]

Fixes None

Summary/Motivation:

This PR allows users of the parameter sweep tool to specify a value for an IndexedVar as a whole, rather than individual elements. All elements will be set to the same value.

Changes proposed in this PR:

• Support for indexed variables in parameter sweep tool
• Clean up a few unused imports in other test files

Legal Acknowledgement

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[bpaul4]

I think this looks nice! Is there a reason some imports were removed in the heat exchanger test files?

[andrewlee94]

@bpaul4 They were left over imports from before I added the DiagnsoticsToolbox tests that were not cleaned up (and pylint is set to ignore test files so it didn't raise any warnings).

[agarciadiego]

LGTM

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.62%. Comparing base (40d3dc7) to head (59888d3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1387      +/-   ##
==========================================
- Coverage   77.63%   77.62%   -0.01%     
==========================================
  Files         391      391              
  Lines       64386    64392       +6     
  Branches    14262    14264       +2     
==========================================
+ Hits        49984    49987       +3     
- Misses      11830    11834       +4     
+ Partials     2572     2571       -1     

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