minor fixes + fixed norm bug in xvalidation

[GTorlai]

Changes implemented in this PR:

• fix the normalization when computing the cost function on the cross-validation data set (in both state and process tomography).
• Add a keyword arg for printing infos about training in tomography.
• add iSwap gate
• add pre-defined circuit to generate the GHZ state
• fix bug in layer sequence for randomcircuit with N=2
• add frobenius_distance for pure states, as well as fidelity_bound (though here if one (or both ) of the two state is pure just returns the fidelity.
• change the syntax circuit(...) to identity_mpo(...).

[codecov-io]

Codecov Report

Merging #166 (2f66db8) into master (1999f56) will increase coverage by 0.04%. The diff coverage is 86.11%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   87.12%   87.17%   +0.04%     
==========================================
  Files          14       14              
  Lines        1600     1637      +37     
==========================================
+ Hits         1394     1427      +33     
- Misses        206      210       +4     

Impacted Files	Coverage Δ
src/circuits/gates.jl	61.01% <0.00%> (-1.06%)	:arrow_down:
src/distances.jl	83.33% <50.00%> (+8.33%)	:arrow_up:
src/circuits/circuits.jl	41.25% <55.55%> (-1.42%)	:arrow_down:
src/circuits/runcircuit.jl	96.46% <83.33%> (ø)
src/processtomography.jl	99.24% <100.00%> (+0.02%)	:arrow_up:
src/statetomography.jl	100.00% <100.00%> (ø)

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