Advanced noise and exact simulator

[GTorlai]

This PR applies the following changes:

• Refined exact simulation of unitary and noisy circuit, which is now included in the general interface with the keyword argument full_representation = true.
• Added sampling of dense wavefunctions and density matrices.
• Modify the convention on gates constructors with argument N. Before N was the dimension of the gate matrix, now it is the number of qubits.
• Introduced n-qubit Pauli channels.
• The noise is now applied as any other gate, where if the noise act on a pair of qubits, the two-qubit channel is invoked (if defined).
• One can add gate-specific noise.

[codecov-commenter]

Codecov Report

Merging #202 (9c79e15) into master (f488809) will increase coverage by 1.54%. The diff coverage is 80.74%.

@@            Coverage Diff             @@
##           master     #202      +/-   ##
==========================================
+ Coverage   80.63%   82.17%   +1.54%     
==========================================
  Files          20       21       +1     
  Lines        2122     1964     -158     
==========================================
- Hits         1711     1614      -97     
+ Misses        411      350      -61     

Impacted Files	Coverage Δ
src/PastaQ.jl	100.00% <ø> (ø)
src/deprecated.jl	0.00% <ø> (ø)
src/lpdo.jl	74.68% <0.00%> (+2.77%)	:arrow_up:
src/circuits/getsamples.jl	88.14% <20.00%> (-11.14%)	:arrow_down:
src/itensor.jl	35.71% <66.66%> (-9.75%)	:arrow_down:
src/utils.jl	77.93% <74.19%> (-1.77%)	:arrow_down:
src/circuits/runcircuit.jl	94.36% <91.42%> (+9.84%)	:arrow_up:
src/circuits/noise.jl	91.89% <91.89%> (ø)
src/circuits/circuits.jl	98.03% <100.00%> (+1.11%)	:arrow_up:
src/circuits/gates.jl	69.91% <100.00%> (+6.11%)	:arrow_up:
... and 21 more

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