refactor: make getp generic of the parameter container

[AayushSabharwal]

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

Add any other context about the problem here.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (b6bbb66) 25.33% compared to head (599575d) 25.87%.

Files	Patch %	Lines
src/parameter_indexing.jl	85.71%	1 Missing ⚠️
src/state_indexing.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   25.33%   25.87%   +0.54%     
==========================================
  Files           7        7              
  Lines         225      228       +3     
==========================================
+ Hits           57       59       +2     
- Misses        168      169       +1     

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