[ENHANCEMENT] Store unitary as a sparse matrix

[ogauthe]

Currently, the unitary matrix used in permutedims is a Dict. It should be some sparse matrix type, possibly using AxisKeys.jl

[ogauthe]

According to AxisKeys.jl documentation,

Note that indexing is the primary way to access the data. Lookup calls for example i = findfirst(axiskeys(A,1), :left) to convert keys to indices, thus will always be slower. If you want this to be the primary mode of access, then you may want a dictionary, possibly Dictionaries.jl.

So I am not sure we should use this since we only access the matrix elements through keys.