Hamiltonian expectation for density matrices

[stavros11]

Fixes #339.

[codecov]

Codecov Report

Merging #344 (1eae838) into master (85448e5) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #344   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           74        74           
  Lines        12100     12147   +47     
=========================================
+ Hits         12100     12147   +47     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/abstractions/states.py	100.00% <100.00%> (ø)
src/qibo/core/hamiltonians.py	100.00% <100.00%> (ø)
src/qibo/core/states.py	100.00% <100.00%> (ø)
src/qibo/tests/test_hamiltonians.py	100.00% <100.00%> (ø)
src/qibo/tests_new/test_core_states.py	100.00% <100.00%> (ø)
src/qibo/core/cgates.py	100.00% <0.00%> (ø)
...c/qibo/tests_new/test_core_gates_density_matrix.py	100.00% <0.00%> (ø)

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[scarrazza]

Thanks, looks good.