Unflatten: restore orbital structure into elements

[pablosanjose]

This introduces an unexported unflatten(vec, h) that turns a flattened vector (e.g. eigenstate produced by using a flatten blochtype) to another vec´ of length(vec´) = size(h,1) and eltype(vec´) = orbitaltype(h). That is, unflatten restores the eigenstate that would have been obtained without flattening the Hamiltonian.

This is required to make progress in generalizing #111 to multiorbital Hamiltonians.

[codecov-io]

Codecov Report

Merging #112 into master will increase coverage by 0.51%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
+ Coverage   60.89%   61.40%   +0.51%     
==========================================
  Files          16       16              
  Lines        2736     2744       +8     
==========================================
+ Hits         1666     1685      +19     
+ Misses       1070     1059      -11     

Impacted Files	Coverage Δ
src/plot_vegalite.jl	0.00% <0.00%> (ø)
src/hamiltonian.jl	75.56% <94.11%> (+0.48%)	⬆️
src/tools.jl	56.48% <100.00%> (+0.55%)	⬆️
src/bandstructure.jl	88.39% <0.00%> (+0.55%)	⬆️

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[pablosanjose]

Also maybe_unflatten that makes no unaliasing guarantees. It will call unflatten (and hence allocate a new array) only if one cannot avoid copying. For uniform number of orbitals it is possible to call reinterpret instead. Also, if unflatten is a no-op, maybe_unflatten will simply return the same input array.