Cache flux BCs

[charleskawczynski]

This PR caches flux boundary conditions, for

• dif_flux_uₕ
• dif_flux_energy
• dif_flux_ρq_tot
  to address this comment (Operators.SetValue(.-dif_flux_uₕ) is allocating).

Note that it seems that the coupler may be impacted by this, due to the coupled flag. cc @LenkaNovak

A step towards #686.

[szy21]

Yes, this will affect the coupler I think. But it should be straightforward to change - just updating e.g. p.dif_flux_uₕ_bc instead of p.dif_flux_uₕ in the coupler.

[charleskawczynski]

bors r+

[bors]

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