Add OrbitalSliceArray

[pablosanjose]

Closes #232

We define a new type of OrbitalSliceArray, following the discussion in #232, that is designed to hold (in principle dense) matrices defined over the orbitals in a given OrbitalSlice. This is now the default output of fully evaluating a Green function, and of observables such as ldos (which gives an OrbitalSliceVector) and densitymatrix (which gives an OrbitalSliceMatrix). The orbital axes of an m::OrbitalSliceArray can be obtained with orbaxes(m), and are in general a tuple of OrbitalSliceGrouped (which represents groups of orbitals over a lattice grouped by sites).

For non-orbital dimensions these orbaxes could also be Missing. This possibility was brought up by @BacAmorim in #232

One critical property of this PR is that view(::OrbitalSliceMatrix, cellsites(cell1, i::Int), cellsites(cell2, j::Int)) is a non-allocating view of the underlying matrix. This allows us to compute a densitymatrix(...)::OrbitalSliceMatrix, and use it as a parameter for a mean field model, which will now be able to do fast lookups over this OrbitalSliceMatrix for individual sites to update a ParametricHamiltonian in-place, hence implementing a self-consistent mean field efficiently. The remaining changes required to close #229 are now relatively simple.