Add convert requirement in check_state

[goerz]

This adds a requirement in check_state that convert(typeof(state), mutable_state) for mutable_state = similar(state) must be defined.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 90.2%. Comparing base (d6dfe6f) to head (17916cf).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/interfaces/state.jl	87.5%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #108     +/-   ##
========================================
- Coverage    90.2%   90.2%   -0.0%     
========================================
  Files          35      35             
  Lines        2550    2558      +8     
========================================
+ Hits         2300    2307      +7     
- Misses        250     251      +1     

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