feat: add IsTimeseriesTrait, support timeseries objects in getu

[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: 34 lines in your changes are missing coverage. Please review.

Comparison is base (9c5e3b5) 0.00% compared to head (29d692b) 24.85%.

Files	Patch %	Lines
src/state_indexing.jl	0.00%	30 Missing ⚠️
src/parameter_indexing.jl	60.00%	2 Missing ⚠️
src/symbol_cache.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #25       +/-   ##
===========================================
+ Coverage    0.00%   24.85%   +24.85%     
===========================================
  Files           5        6        +1     
  Lines         150      177       +27     
===========================================
+ Hits            0       44       +44     
+ Misses        150      133       -17     

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