debug: reset prepared flag in initstate!(::StateEstimator) method

[franckgaga]

This is a minor debug to ensure that the user does not call updatestate! w/o preparestate! when direct==true just after calling initstate! on a StateEstimator (or a PredictiveController). By resetting the flag, an error will be thrown in such cases, as it should be to guide the user.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.40%. Comparing base (86844fc) to head (4540349).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #416   +/-   ##
=======================================
  Coverage   98.40%   98.40%           
=======================================
  Files          29       29           
  Lines        6130     6132    +2     
=======================================
+ Hits         6032     6034    +2     
  Misses         98       98           

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