add algebraic trick for ipu_eigh initialisation

[akrzgc]

The change:

• adds algebraic trick with the ipu_eigh initialization
• adds ipu_eigh_its parameter so that we can now select the number of ipu_eigh iterations from CLI
• adds visualisation of eigenvectors for each nanoDFT iteration which is created as animated gif when new parameter --visualise_eigvects true

[AlexanderMath]

Looks great! Minor comments: (1) some stuff wrt naming and hinting on linear algebra stuff (likely not too useful) and (2) potential performance issue with at[i].set(.) vs jax.lax.dynamic_update_slice(.) (could you try to run e.g. benzene on IPU with both, if the cycle count is +-5% my concerns are misplaced).

[awf]

We need a before/after graph or table to show the effect of the change, all other things being fixed

[akrzgc]

We need a before/after graph or table to show the effect of the change, all other things being fixed

Do you mean before/after of ipu_eigh input? Should I attach it to this review or would you like it to be plotted each time nanoDFT.py is run?