Simple bypass to allow APs to be unitless with unitful connections

[cmhyett]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Should close #3842, albeit with a solution for a different problem than stated there originally.

[cmhyett]

Unit checking logic is duplicated between src/systems/unit_check.jl and src/systems/validation.jl I'm not sure if this is intended or not. It seems unit_check.jl and validation.jl are perhaps used in different instances?

Regardless, the logic for the fix is simple

• Previously MTK was trying to check units on an AnalysisPoint, but didn't have a method defined.
• AnalysisPoints can be thought of as just connections with additional data so I added a simple passthrough
  • When checking units on an AP, remake the connection without the AP and use existing logic to check the units
• Added a test to check this functionality is hit. (The test actually calls the validation.jl version, again, unsure if the unit_check.jl is ever used, but seemed prudent to include the logic there as well).

@ChrisRackauckas ready for your review; I believe the test failures are coming from the main branch, please advise if you'd like me to rebase this on an upstream version.

[AayushSabharwal]

You'll need to import MTKStdlib.Blocks for this test. It's currently failing in CI.

[cmhyett]

Sure, I think the submodule is already loaded (twice in fact!), so I just removed the Blocks namespacing. I'll keep an eye on the tests and ping this when it is resolved.

[cmhyett]

CI test failure is pointing here (and other uses of Gain below, not the testsets I added above); Gain is exported by two modules, MTKStdLib.Blocks and...something else? and the ambiguity is causing the error. Unsure why it's throwing after my changes, but let me see if I can specify the module to fix.