Fix bug in ptranspose and added corresponding tests

[zejian-li]

fixed bug reported in #97
the ptranspose(rho::DenseOpType{B,B}, indices::Union{Vector{Int},Tuple{Vararg{Int}}}) method is added to support multi-index subsystems, and the original single-index method ptranspose(rho::DenseOpType{B,B}, index::Int=1) is kept for compatibility, but implemented as a special case using the multi-index version.
tests dedicated to the two methods are added.

[codecov]

Codecov Report

Merging #98 (72bd2c3) into master (1953938) will decrease coverage by 3.63%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #98      +/-   ##
==========================================
- Coverage   96.65%   93.03%   -3.63%     
==========================================
  Files          26       24       -2     
  Lines        3410     3159     -251     
==========================================
- Hits         3296     2939     -357     
- Misses        114      220     +106     

Impacted Files	Coverage Δ
src/metrics.jl	100.00% <100.00%> (ø)

... and 25 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[david-pl]

@zejian-li Thanks for this! It looks fine, I just have one small question: any reason you added another dispatch for index::Int ? In Julia, integers are iterable, so the method for ptranspose(rho::DenseOpType{B,B}, indices::Union{Vector{Int},Tuple{Vararg{Int}}}) also works for indices::Int if you just remove the typing, i.e.make the signature ptranspose(rho::DenseOpType{B,B}, indices). Then you can remove the additional ptranspose(rho::DenseOpType{B,B}, index::Int) dispatch altogether.

[zejian-li]

@david-pl yes you are right! I will change the signature back to ptranspose(rho::DenseOpType{B,B}, indices=1), i.e. the original signature for the current implementation in the master branch, for compatibility.

[zejian-li]

Should be good now. The negativity functions will also support now multiple indices without modification.