Fix leftsite and rightsite for adjoint Chains

[jofrevalles]

Previously, doing leftsite or rightsite for a Chain that is adjoint, returned a Site that does not have dual=true. This caused the functions leftindex and rightindex to not work for adjointed Chains.

Example (code before the PR):

julia> using Qrochet

julia> mps = rand(Chain, Open, State; n=8, χ=10)
MPS (inputs=0, outputs=8)

julia> Qrochet.rightsite(mps', Site(2; dual=true))
3

julia> rightindex(mps', Site(2; dual=true))
ERROR: AssertionError: Site 3 not found
Stacktrace: ...

julia> rightindex(mps', Site(2))
ERROR: AssertionError: Site 2 not found
Stacktrace: ...

This PR addresses this issue by fixing the leftsite or rightsite functions. Additionally, we added the testset for the adjoint for Chains.

Example (fixed code):

julia> using Qrochet

julia> mps = rand(Chain, Open, State; n=8, χ=10)
MPS (inputs=0, outputs=8)

julia> Qrochet.rightsite(mps', Site(2; dual=true))
3'

julia> rightindex(mps', Site(2; dual=true))
Symbol("Z'")