fix: change variable defaults to guesses

[avinashresearch1]

Checklist

• [ ] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] 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

Add any other context about the problem here.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (3a8fb0a) to head (6e2af95). Report is 7 commits behind head on main.

:exclamation: Current head 6e2af95 differs from pull request most recent head f000b3d

Please upload reports for the commit f000b3d to get more accurate results.

Files	Patch %	Lines
src/Blocks/utils.jl	0.00%	29 Missing :warning:
src/Blocks/continuous.jl	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #281 +/- ## ========================================== - Coverage 61.48% 0.00% -61.49% ========================================== Files 46 32 -14 Lines 1472 1657 +185 ========================================== - Hits 905 0 -905 - Misses 567 1657 +1090 ```

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[avinashresearch1]

@ven-k I'm a little confused as to what is causing the test failure, its from the code below:

@mtkmodel SISO begin
           @parameters begin
               u_start = 0.0
               y_start = 0.0
           end
           @variables begin
               u(t), [guess = u_start, description = "Input of SISO system"]
               y(t), [guess = y_start, description = "Output of SISO system"]
           end
           @components begin
               input = RealInput(u_start = u_start)
               output = RealOutput(u_start = y_start)
           end
           @equations begin
               u ~ input.u
               y ~ output.u
           end
       end

its due to guess = u_start, not sure why it says u_start not defined when I'm trying to use the one in the params (I'm not extremely familiar with the @mtkmodel syntax).

[avinashresearch1]

@ven-k I'm a little confused as to what is causing the test failure, its from the code below:

@mtkmodel SISO begin
           @parameters begin
               u_start = 0.0
               y_start = 0.0
           end
           @variables begin
               u(t), [guess = u_start, description = "Input of SISO system"]
               y(t), [guess = y_start, description = "Output of SISO system"]
           end
           @components begin
               input = RealInput(u_start = u_start)
               output = RealOutput(u_start = y_start)
           end
           @equations begin
               u ~ input.u
               y ~ output.u
           end
       end

its due to guess = u_start, not sure why it says u_start not defined when I'm trying to use the one in the params (I'm not extremely familiar with the @mtkmodel syntax).

This requires: https://github.com/SciML/ModelingToolkit.jl/pull/2501

[avinashresearch1]

Rebased after @AayushSabharwal 's changes #291

[avinashresearch1]

@baggepinnen request for review

[ChrisRackauckas]

Where is this at?