This PR makes GD paralellize over sites

[Gertian]

To remarks :

1. I failed to paralelize

function retract(x::ManifoldPoint{<:FiniteMPS}, g, alpha)
    #TODO : support parralelize_sites.
    state = x.state
    envs = x.envs

    y = copy(state)  # The end-point
    h = similar(g)  # The tangent at the end-point
    for i in 1:length(g)
        yal, th = Grassmann.retract(state.AL[i], g[i].Pg, alpha)
        h[i] = PrecGrad(th)
        y.AC[i] = (yal, state.CR[i])
    end
    normalize!(y)

    n_y = ManifoldPoint(y, envs)

    return n_y, h
end

2)I don't think there are explicit tests for multi-site unit cells. Is this something that should be added ?

[Gertian]

Ok I'm sorry for the mess.
I'm trying to do another PR at the same time so I need to give this one a better name.

In fact I should have done that in the first place...

Remaking this with better name :)

[codecov]

Codecov Report

Attention: Patch coverage is 76.19048% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/algorithms/grassmann.jl	76.19%	10 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/grassmann.jl	92.95% <76.19%> (-7.05%)	⬇️

... and 2 files with indirect coverage changes