Update operator interface

[goerz]

Implement size and eltype, as required by JuliaQuantumControl/QuantumPropagators.jl#98

[codecov]

Codecov Report

❌ Patch coverage is 63.15789% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.0%. Comparing base (27cc659) to head (88da7d0).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/linalg.jl	56.3%	7 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##           master     #17   +/-   ##
======================================
  Coverage    80.0%   80.0%           
======================================
  Files           5       5           
  Lines         190     185    -5     
======================================
- Hits          152     148    -4     
+ Misses         38      37    -1     

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