Add traits for matrix interfaces

[goerz]

A lot of external packages are happy if operators and states are abstract arrays. We can make that part of the interface.

[codecov]

Codecov Report

❌ Patch coverage is 95.38462% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.2%. Comparing base (8609179) to head (6cd2b0f).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/interfaces/operator.jl	97.8%	2 Missing ⚠️
src/interfaces/state.jl	98.0%	2 Missing ⚠️
src/interfaces/supports_matrix_interface.jl	33.4%	2 Missing ⚠️
src/interfaces/supports_vector_interface.jl	33.4%	2 Missing ⚠️
src/interfaces/supports_inplace.jl	0.0%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #100     +/-   ##
========================================
+ Coverage    87.0%   89.2%   +2.3%     
========================================
  Files          33      35      +2     
  Lines        2327    2515    +188     
========================================
+ Hits         2023    2243    +220     
+ Misses        304     272     -32     

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